There aint only one simple answer here.
I've picked natural whole foods and ate until i hate life, and gained significant weight. Just writing down something very rough that can shape your mind
Get 3 solid meals a day, each consisting plenty of;
.Starch (potatoes/rice/oats/ maybe even pasta or bread)
. Fat (if the meat is lean)
. A portion of Veg
And have snacks/ shakes in between. I'd go for shakes since liquid is easier to go down. The options here are endless, play with whole milk/yogurt, protein powders, nut butters, raw eggs, bananas, honey etc.
If you're not counting every calories (which is understandable), then eat until you're weight is going up as desired. Keep in mind that going up too quick is probably fat gain and might slow your boxing. Roughly 300-400 above maintenance is good. As an ex athlete i'd say dont be obsessed with weight but performance, as long as youre getting strong its prob fine.