Spread tomatoes onto a cookie sheet and combine with garlic, shallot, 3 tbsp olive oil, salt and pepper. The tomatoes will burst after 10 minutes. Continue cooking another 10-12 minutes to concentrate flavor.
About 10 minutes before you are ready to eat, season fish with salt and pepper and heat 2 tablespoons olive oil in a skillet over medium-high heat. If your fish has skin attached to one side, place skin-side down first. Cook 2 minutes on each side, then transfer to the oven for 5 minutes.
Remove tomatoes from oven. Mash the tomatoes with a potato masher on the cookie sheet. Top fish with roasted tomato sauce.