In a bowl, mix beef, turkey, diced onion, bread crumbs, garlic salt and tomato paste. In another bowl, mix egg white, ricotta, spinach, salt and pepper.
Preheat oven to 350°F. Turn out the beef mixture onto a large sheet of wax paper, and form it into a 9×10-in. rectangle with your hands.
Spoon the spinach stuffing lengthwise down the centre of the meat, leaving about 1 in. of it uncovered at each short end.
With the help of the wax paper, lift the long edges of the meat. Fold meat over stuffing to enclose it. Slip wax paper out and discard.
Using your fingers, pinch the edges of the meat together. Place loaf seam-side down in a non-stick roasting pan. Add sliced onions, chopped carrots and tomatoes to pan.
Bake for about 11/2 hours, or until meat and vegetables are cooked through. Transfer meat to a platter. Purée vegetables in a blender and serve this vegetable sauce with the meat loaf.
Recipe Notes
Per serving: 294 calories, 32 g protein, 28 g carbohydrates, 6 g total fat (2 g saturated fat), 71 mg cholesterol, 2 g fibre, 405 mg sodium.