1. Bring 4 inches water to boil in a large pot. Add lobsters, cover and cook 12 minutes. Transfer lobsters to colander and cool completely. Carefully remove meat from shells. Discard all shells.
2. Remove grapefruit peel (yellow part only) with a vegetable peeler; cut peel into thin strips. Cut away white pith from grapefruit; discard pith. Cut out grapefruit segments from membrane and discard membrane; set aside.Make Citrus Butter::
3. Meanwhile, bring grapefruit juice, broth, and cream to boil in heavy saucepan over medium heat; boil until reduced to 1/3 cup. Reduce heat to very low. Whisk in butter 1 tablespoon at a time. Makes 1/2 cup. Keep warm (sauce will separate if overheated).Make Glazed Onions::
4. Cook onions in boiling salted water to cover just until tender, 5 minutes. Rinse under cold water and peel. Combine onions, butter, sugar, salt, and pepper in small saucepan and cook over medium heat, swirling occasionally, until browned, 5 to 10 minutes.
5. Heat grill or broiler. Cut lobster tails in half lengthwise. Brush lobsters with 2 tablespoons Citrus Butter and grill or broil 4 inches from heat 30 seconds per side.To serve::
6. Spoon orzo onto 2 heated dinner plates and top with lobster. Spoon 1 more tablespoon Citrus Butter around each lobster. Arrange Glazed Onions and Spinach Saute (if using) around orzo. Garnish with reserved grapefruit sections and peel strips, if desired. Makes 2 servings.