2 8 ounce packages dried cheese-filled tortellini
3 tablespoons finely chopped red onion
2 tablespoons finely chopped shallots
1 tablespoon butter
2 cups half-and-half or light cream
1/2 cup milk
2 egg yolks
1/4 cup grated Parmesan cheese
1 tablespoon snipped fresh sage or 1 teaspoon dried sage, crushed
1/4 teaspoon ground black pepper
1/2 cup shredded Gruyere or Swiss cheese (2 ounces)
1/2 cup walnut pieces
2 ounces prosciutto, finely snipped, or cooked ham, finely chopped