Preheat oven to 350ºF.
Cut ½ inch off of the stem side from each tomato and scoop out insides.
Place insides in food processor.
Put tomatoes cut side down on paper towel-lined plate.
Heat oil in large skillet over medium heat.
Add onions and bell pepper, cook for 4-5 minutes or until softened.
Add spinach and stir until wilted.
Mix in corn, black beans and ¾ cup cheese.
In small bowl, mix cornstarch with 2 tablespoons water until smooth.
Add cornstarch mixture, tomato paste, chili powder, salt and 1 cup water to food processor.
Puree until smooth.
Pour ¾ of sauce into baking pan.
Fill tomatoes with bean mixture and place in pan.
Cover with remaining sauce and sprinkle with remaining cheese.
Bake for 30 minutes, or until cheese has melted and sauce has thickened.