Heat the oil in a12-inch skillet over medium heat. Add the onion and cook, stirring, until softened, about 2 minutes. Add the tomatoes, zucchini, bell pepper, garlic, 2 tsp. salt, and 1/2 tsp. pepper and cook, stirring occasionally, until the vegetables are barely tender, about 7 minutes. Pour in the heavy cream and simmer until it thickens and coats the back of a spoon, about 7 minutes. Stir in the cheese, basil, and lemon zest.