Set oven rack about 6 inches from the heat source and preheat the oven's broiler.
Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally until cooked through but firm to the bite, 8 minutes. Drain and transfer pasta to a large baking dish.
Heat vegetable oil and flour in a large skillet over medium heat; cook and stir until mixture is frothy and smooth, about 5 minutes. Add reserved tomato juice and garlic paste. Cook and stir until mixture is thickened and stir in chopped tomatoes and about 3/4 the Cheddar cheese. Simmer until cheese is melted, about 5 minutes more. Stir water into tomato sauce if mixture becomes too thick.
Pour tomato sauce over macaroni and top with remaining Cheddar cheese and sliced tomato.
Broil in the preheated oven until cheese is melted and browned, about 5 minutes.