Francisco Alvarez hits first spring homer in encouraging Mets sign

Alvarez belted 25 home runs — the most by a catcher at age 21 or younger since Johnny Bench’s 26 in 1969 — during a wildly impressive if offensively uneven rookie campaign last year.

The once No. 1 prospect in all of baseball brought the power but hit just .209 with a .721 OPS. When he was hot, he sizzled (such as a blistering May in which he crushed seven homers and posted a 1.029 OPS), and when he was cold, he was frigid (such as a rough .456-OPS August).

In his first game of the spring, Alvarez looked comfortable immediately.

He smashed what became an infield single down the third-base line against righty Brandon Bielak in the first inning and then drilled a top-of-the-zone fastball 354 feet into the right-field seats in his next and final at-bat.

If Alvarez’s bat steadies, his ceiling might as well be a cathedral ceiling.

Alvarez has declined to cite a goal for how many homers he can hit, focusing on wins and not wanting to put a cap on the dinger total.

All along last season, his defense — which arrived with more questions than his bat — impressed. Alvarez was among the league’s best at framing pitches and threw out 13 percent of base runners, a stat he would like to improve upon.

Alvarez said he was hoping an Astros base runner would attempt a steal while he was in the game, but none did.

First-year manager Carlos Mendoza enjoyed the swing but also was pleased with the way Alvarez caught the Mets’ starting pitcher.

“The way he worked with [José] Butto was really good to see,” Mendoza said.