Cook pancetta or bacon in a 4-quart Dutch oven till browned. Remove with a slotted spoon, reserving drippings in pan. Drain on paper towels; cover and chill. Add mushrooms, leeks, and garlic to the Dutch oven; cook and stir about 3 minutes or till mushrooms and leeks are tender.
Carefully add potatoes, broth, and 1/4 teaspoon pepper to Dutch oven. Bring to boiling. Reduce heat and simmer, covered, about 15 minutes or till potatoes are tender. Add shredded carrot.
To serve, thaw shrimp and scallops, if frozen. Stir together melted margarine and flour. Add flour mixture and half-and-half to Dutch oven. Cook and stir over medium heat till bubbly. Add shrimp, scallops, and pancetta. Cook and stir 2 to 3 minutes more or till shrimp are pink and scallops are opaque. Ladle into bowls. If desired, garnish with lemon slices and/or thyme.