Step 1
Preheat the oven to 375°F. Bring a large pot of lightly salted water to a boil. Add the macaroni and cook, stirring occasionally, until slightly softened but still firm, about 3 minutes. Drain the macaroni and set aside.
Step 2
Return the pot to medium heat, add the butter, and when it has nearly melted, whisk in the flour to make a smooth paste. Add the milk and cook, whisking frequently, until the sauce has thickened and coats the back of a spoon, about 10 minutes. Remove from the heat and stir in the spinach and 1½ cups of goat cheese. Season the sauce to taste with salt and black pepper, then stir in the reserved macaroni. Transfer to an 8-by-8-inch baking dish, layer the tomato slices evenly over the surface, then sprinkle over the remaining goat cheese.