Place the scallops in a saucepan along with ⅓ cup water, half of the lemon juice, and add the onion, peppercorns, and bay leaf for aromatic flavor.
Bring the liquid to a gentle simmer over medium-low heat, then cover the pan with a lid. Allow the scallops to poach very gently for 2–3 minutes, just until they turn opaque and tender—be careful not to overcook them.
Using a slotted spoon, carefully lift the scallops from the poaching liquid and set them aside on a plate. Strain the cooking liquid through a fine-mesh sieve to remove the solids, reserving the flavorful liquid for the sauce.
To prepare the sauce, melt the butter in a clean saucepan over medium heat. Add the mushrooms and cook gently, stirring occasionally, for about 2 minutes until they begin to soften. Sprinkle the flour over the mushrooms and stir continuously for 1 minute to cook out the raw flour taste. Remove the pan from the heat and gradually blend in the reserved strained poaching liquid, whisking to prevent lumps. Return to the heat and cook, stirring constantly, for 1 minute until the sauce thickens smoothly.
Stir in the cream and tomato paste, mixing well to combine. Bring the sauce to a gentle boil, stirring constantly as it thickens and becomes velvety. Let it simmer for 2 minutes to develop the flavors, then season with salt and pepper according to your taste.
Meanwhile, cook the pasta shells in a large pot of boiling salted water for 8–10 minutes, or until they’re perfectly tender. Drain the pasta thoroughly in a colander, then return it to the warm pot and toss with the butter and the remaining lemon juice until well coated. Gently fold the reserved scallops into the warm sauce and heat through very gently over low heat, being careful not to toughen the delicate seafood.
Arrange the buttery pasta shells on warmed serving plates, creating an attractive base. Generously spoon the creamy scallop sauce over the top of each portion. Garnish with fresh parsley and thin lemon slices for a bright finishing touch, and serve immediately while piping hot.