Preheat the oven to 375F and prepare the baking dish with cooking spray.
Arrange the halved cherry tomatoes, cut side up, in a single layer in the baking dish
Scatter the chopped onions on top of the tomatoes.
Put the bread pieces in the bottom of the food processor and add in the butter and seasonings.
Pulse the food processor until the mix resembles crumbs and the fat and seasonings are well distributed. You might need to stop and scrape down the sides or add more fat, especially if using olive oil.
Spread the crumbs over top the tomatoes and onions. You might not need them all.
Bake for 15 - 20 minutes or until the tomatoes are heated through and the crumbs are golden brown.