Without a doubt, really great tomatoes will be much better in this recipe. But, I've made this baked dinner throughout the year (actually, most often when tomatoes aren't at their peak). And I feel like there are enough other big flavors going on, so that you can sneak by with winter tomatoes, as long as they're halfway decent. When regular tomatoes aren't looking great, though, you might look to substitute an equivalent amount of Roma (plum) tomatoes, or even little grape or cherry tomatoes, which tend to be more reliably flavorful all year 'round in grocery stores these days. Just remember to remove the seeds, so your caprese topping has the best texture.