Press all liquid from tuna. Crumble the tuna into a bowl. Add the hummus, spinach, carrots and cumin, then mix thoroughly. Make four sandwiches with the bread and tuna mixture.
In a shallow bowl, whisk together the milk, egg and egg whites. Dip one side of each sandwich at a time into the egg mixture until it is lightly soaked. You should use up all of the egg mixture.
In a large nonstick skillet (or two medium nonstick skillets) over very low heat, heat oil. Add the sandwiches and cover the skillet loosely with a lid or piece of foil.
Cook until the undersides of the sandwiches are well-browned, 8 to 10 minutes. Turn the sandwiches over and cook, still covered, until browned on the second side, about 8 minutes more.
Nutrition information per serving: 324 calories; 9 g fat (2 g saturated); 62 mg cholesterol; 37 g carbohydrate; 25 g protein; 7 g fiber; 662 mg sodium.