For the filling:
2 tbsp. extra virgin olive oil
2 tbsp. unsalted butter
2 medium yellow onions, thinly sliced
4 cloves garlic, minced
1 tsp. fresh thyme
3 lbs. cherry or grape tomatoes
3 tbsp. all-purpose flour
¼ tsp. red pepper flakes
1½ tsp. coarse salt
Pinch of black pepper
For the biscuit topping:
2 cups all-purpose flour
2 tsp. baking powder
1 tsp. salt
8 tbsp. cold unsalted butter, cut into pieces
1 cup grated gruyere cheese, plus extra for sprinkling
1½ cups heavy cream
To prepare the filling, combine the oil and butter in a large skillet over medium heat. Add the onions and cook, stirring occasionally, until cooked down and caramelized, about 20-25 minutes. Stir in the garlic and thyme and cook until fragrant, 1-2 minutes more. Remove from the heat and let cool.
Preheat the oven to 375˚ F. In a large bowl, combine the onion mixture, tomatoes, flour, red pepper flakes, salt and pepper. To make the biscuit topping, combine the flour, baking powder and salt in a medium bowl. Cut in the butter with a pastry blender or two knives until the mixture is crumbly and small clumps form. Mix in the cheese. Stir in the cream with a fork until a sticky dough forms and the dry ingredients are incorporated.
Transfer the tomato mixture to a 9 x 13-inch baking dish. Using a large ice cream scoop or a ¼-cup measure, drop clumps of the biscuit dough evenly over the tomato mixture. Sprinkle the top with additional grated cheese. Bake until the tomatoes are bubbling and the biscuits are golden, about 1 hour and 10 minutes. Let cool 20 minutes before serving.