Birthday Lasagna

Name: Birthday Lasagna
Ingredients: 3 tablespoons unsalted butter; 3 tablespoons all-purpose flour; 1 1/2 cup whole milk; Salt; Pinch freshly ground nutmeg; 1/2 package no-boil (or the equivalent in fresh) lasagna sheets; 3/4 cups shredded fresh mozzarella; 2 tablespoons olive oil; 1 pound ground beef (85% lean); 1/2 cup chopped carrot; 1/2 cup finely chopped onion; 1 clove garlic, chopped; 1 teaspoon tomato paste; 2 teaspoons all-purpose flour; Splash red wine (optional); 1 1/2 cup canned chopped tomatoes, with their juices; 1 cup beef stock or water; 1/2 teaspoon chopped fresh oregano