Preheat oven to 350°F.

Cut 1/2 inch off the stem side of each tomato and scoop out the insides.

Place the tomato insides in a food processor.

Put the tomatoes cut-side down on a paper towel-lined plate to drain.

Heat olive oil in a large skillet over medium heat.

Add chopped onions and cook for 4-5 minutes, or until softened.

Add baby spinach and stir until wilted.

Mix in corn, black beans, and 3/4 cup of shredded cheddar cheese.

In a small bowl, mix cornstarch with 2 tablespoons of water until smooth.

Add cornstarch mixture, tomato paste, chili powder, salt, and 1 cup of water to the food processor.

Puree until smooth.

Pour 3/4 of the sauce into a baking pan.

Fill the tomatoes with the bean mixture and place them in the pan.

Cover with the remaining sauce and sprinkle with the remaining cheese.

Bake for 30 minutes, or until the cheese has melted and the sauce has thickened.