MIAMI — On Monday night, Pete Alonso provided the first of what likely will be regular reminders of why it was so wise of the Mets to bring him back for at least another season.

In their 10-4 win over the Marlins, Alonso highlighted the homer-palooza with a game-turning grand slam in the fifth inning.

The first long ball in each new season, he said, feels different from the rest.

“It’s a long time without hitting one,” Alonso said. “To finally do it in a big-league game feels pretty cool. I can do it still.”

Brandon Nimmo, leadoff-man-for-a-day Starling Marte and Luis Torrens also went deep in the sort of offensive explosion that the Mets never came close to in their season-opening series against the Astros.

Against a Miami roster featuring few players with any sort of major-league track record of success, most of the Mets came away with reason to feel good about themselves — and all of the starters came away with a hit.

Alonso’s was the biggest. The Mets had just gone ahead 2-1 on Jose Siri’s RBI double when Marte got hit by a pitch and Juan Soto walked. That loaded the bases for Alonso, who smashed Cal Quantrill’s full-count sinker over the right-centerfield wall.

“He earned that pitch that he ended up hitting,” manager Carlos Mendoza said. “Laid off a slider, first pitch. They’re trying to make him chase. Then they painted a fastball at the knees to try to get a ground ball. And then finally he got a pitch he was able to do some damage on and got the big one.”

Alonso said: “He tried to throw his best off-speed stuff. I did a good job of laying off those pitches, those breaking pitches, and worked my way into getting a fastball. I got it over the plate and hit it hard.”

With 227 home runs in his career, Alonso is 25 shy of matching Darryl Strawberry for the most in franchise history.

Quantrill, who signed a one-year, $3.5 million contract in mid-February that represented the Marlins’ largest financial outlay of the offseason, pitched around trouble before imploding in that fifth. In the Mets’ seven-run frame, he gave up six runs and threw 28 pitches without getting an out.

Torrens got in on the fun with a fly ball to centerfield. Marlins outfielder Derek Hill nearly caught it, but his collision with the wall seemed to jar the ball loose. It fell out of his glove and on the other side of the wall. A confused, shrugging Torrens stopped between first and second base for a few seconds until the also-confused umpires signaled home run.

“He helped that one a little bit, but it’s part of the game,” Torrens said through an interpreter.

Mets lefthander David Peterson turned in a quality start, allowing two runs in six innings. Otto Lopez homered in the first and Eric Wagaman went deep in the sixth. Peterson struck out nine and scattered three walks and five hits.

His greatest escape came in the second inning before he settled into a groove. He walked the first two batters — Jonah Bride and Dane Myers — and had to deal with two runners in scoring position after Liam Hicks’ sacrifice bunt. He then struck out Javier Sanoja and Graham Pauley, two of the Marlins’ four rookies, to escape the jam.

After the Mets blew it open, Peterson began the bottom of the fifth by throwing nine of his first 10 pitches for strikes.

“That’s what you want to see,” Mendoza said. “Especially when you have [the kind of] inning that we had, you want your pitcher to go out there and attack, give us a shutdown inning.”

The offensive support proved to be plenty.

“We know we have a good offense,” Mendoza said. “You’re going to go at times through stretches when it’s going to be hard. You’re going to be facing some good pitching staffs. That was the case against Houston. [Tuesday] we got [Sandy] Alcantara. So we gotta bring it.”

Notes & quotes: Francisco Lindor’s wife, Katia, gave birth to their third child and first son, Koa, on Sunday. “Perfect timing,” Mendoza said of the baby coming on the team’s off day. Lindor was available off the bench, but the Mets wound up not needing him in the easy win . . . The Mets traded outfielder Alexander Canario to the Pirates for cash . . . On MLB’s list of most popular jerseys, based on offseason sales, the Mets had two players: Soto at No. 3 and Lindor at No. 6.