Preheat oven to 350 degrees.
To make the tomato filling melt 2 tablespoons butter over medium-high heat in a large oven-proof skillet. (See NOTE below) Add the chopped onion and cook, stirring occasionally, until soft and light brown, about 5 minutes. Add the garlic, thyme and cayenne pepper. Cook and stir for 1 more minute then add the chopped tomatoes, brown sugar and 1 1/4 teaspoons salt.
Bring to a simmer and cook just until the tomatoes start to soften, 4-5 minutes. Do not overcook. Remove the skillet from the heat and gently stir in the flour and cherry tomatoes. Dot with the remaining 1 tablespoon of butter and set aside.
To make the biscuit topping whisk together the flour, baking powder, granulated sugar, 3/4 teaspoon salt and black pepper in a medium bowl. Add the sliced butter and using a pastry cutter, blend the butter into the flour until it resembles coarse meal. Add 2/3 cup of milk, the thyme and mustard to the flour mixture. Gently stir with a fork just until a sticky dough forms. To do over mix.
Using an ice cream scoop, drop balls of dough over the filling. Brush the dough balls with milk and bake until the biscuits are golden brown and the filling is bubbly - about 55-60 minutes. Allow the cobbler to rest about 15 minutes before serving.
Serves 6 (or two at my house)