Cook baby pumpkins in 1/2 inch boiling water in a Dutch oven, covered, 20 minutes or until tender; drain. Cut off and reserve tops. Peel pumpkins, and cut in half. Discard seeds. Mash pulp, and set aside 1 1/4 cups, reserving remainder for another use.
Peel large shrimp, leaving tails on; devein, if desired, and set aside. Peel medium shrimp, and devein, if desired; set aside.
Discard mushroom stems; thinly slice caps.
Stir together fish bouillon cubes and 2 quarts boiling water until bouillon dissolves.
Melt 1/2 cup butter in a Dutch oven over medium heat; add shallots, garlic, and rice, and sauté until shallots are tender.
Add 1 cup hot fish broth; cook, stirring constantly, until liquid is absorbed. Repeat procedure with remaining hot broth, 1 cup at a time. (Cooking time is 35 to 40 minutes.) Add 1 cup mashed pumpkin (reserve remaining 1/4 cup pumpkin for Beurre Blanc Sauce), medium shrimp, sliced mushrooms, Parmesan cheese, and thyme; cook, stirring constantly, 8 to 10 minutes or just until shrimp turn pink.
Melt 1 tablespoon butter in a large skillet; add large shrimp, and sauté 2 to 4 minutes or just until shrimp turn pink. Spoon pumpkin risotto onto serving plates, and drizzle evenly with Beurre Blanc Sauce. Arrange large shrimp around risotto, and top with reserved pumpkin tops.Garnish, if desired.
*1 1/4 cups canned pumpkin maybe substituted for pulp from 6 baby pumpkins.