1. Rinse and dry the scallops. Squeeze the juice of 1/2 lemon and cut 4 slices from the other half of the lemon. Peel and chop the shallot. Rinse and dry the mushrooms, remove the stems and slice the caps.
2. Place the mushrooms in a large non-stick skillet, season with salt, and cook over high heat for 2 minutes to release their moisture: add the lemon juice halfway through cooking. Drain in a colander and set aside.
3. Wipe the skillet clean, heat 1 teaspoon of margarine in it. Sear the scallops over high heat for 2 minutes, turning them halfway through cooking. Season with salt and pepper. Transfer to a plate. Add 1 teaspoon of margarine to the skillet, sweat the shallot for 1 minute, then add the drained mushrooms and cook for 2 minutes over medium heat. Stir in the soy cream, sprinkle with coriander, and mix. Adjust the seasoning.
4. Return the scallops to the skillet and gently mix everything together for 1 minute over low heat. Divide the mixture among the plates. Give it 2 or 3 turns of the pepper mill and garnish with a lemon slice. Serve immediately.