A melted mozzarella center makes these risotto cakes special. Use extra in sandwiches and pasta.
Preheat oven to 450 degrees. Form risotto into 8 balls. Press a cube of cheese into center of each ball and flatten risotto into 1/2-inch-thick patties (make sure cheese is covered). Dredge patties in flour and place on a baking sheet. Refrigerate until firm, 15 minutes.
Meanwhile, on a large rimmed baking sheet, toss tomatoes with thyme and olive oil and season with salt and pepper. Roast until tomatoes begin to collapse, about 20 minutes.
In a large skillet, heat vegetable oil over medium until it sizzles when pinch of flour is added. Dredge patties in flour again (shaking off excess). In two batches, fry patties until golden and warmed through, about 10 minutes per batch. Drain on paper towels. Serve risotto cakes with arugula and tomatoes.