Bring the salted water to a rapid boil. Add the live lobsters and cover, steaming for 5 minutes over very high heat. Remove the lobsters from the salted water and set aside. Reserve the cooking liquid, keeping it heated over medium-low heat.
Remove the lobster meat, and reserve the shells for the stock. Dice the par-cooked meat into small pieces, then cover and refrigerate, reserving for later use.
Add the reserved lobster shells to a large saute pan over high heat with the tomato paste, carrots, celery, onions, and butter. Stir gently until the mixture becomes slightly golden brown. Add the sherry and then pour the mixture into the reserved, warm broth. Add the peppercorns, parsley spigs, and bay leaf. Boil uncovered for 30 minutes. Strain out the solids and keep the stock heated at a low simmer.
Melt the butter in a 4-quart sauce pan and stir in the flour. Cook for 3 to 5 minutes over medium heat, stirring constantly to make a roux. Slowly add the hot lobster stock, constantly whisking to avoid lumps. Add the half-and-half, and simmer gently over medium heat stirring often for about 15 minutes. Add the reserved diced lobster, the sherry, and lemon juice. Adjust the seasoning with salt and white pepper as desired. Garnish with some toasted pumpkin seeds.