Bring a large pot of water to boil. Cook pasta according to package directions. Drain. Do not rinse. Set aside.
Melt butter in a large, high-sided skillet over medium heat. Stir in flour and cook, stirring frequently, until the mixture begins to brown. Whisk in milk.
Reduce heat to medium-low. Stir in garlic powder, onion powder, salt, pepper and nutmeg. Add cheese and lemon juice, stirring to incorporate.
Cook for a couple of minutes until all of the cheese has melted.
Stir in the spinach, working in a couple batches if necessary, until all of it has wilted into the sauce.
Fold in tomatoes and cooked pasta.
Adjust salt and/or pepper to taste.