Preheat oven to 400 degrees. Oil a medium baking dish. In a small skillet, heat oil over medium heat. Add onion and sauté, stirring occasionally, until softened but not brown, about 5 minutes. Add the garlic and cook 1 minute more. Season with salt. Scatter half of the onions on the bottom of the casserole dish. Cover with half of the potatoes and half of the tomatoes; salt lightly. Sprinkle half of the chiles and cheese on top. Repeat the layers, ending with the cheese. Pour the half-and-half evenly over the vegetables. Bake 35 to 40 minutes, or until the liquid has been absorbed and the cheese is brown. Serve at once.