Dredge scallops in flour, shaking off excess, and season with salt and pepper. In large skillet set over moderate flame, heat olive oil and butter until hot. Add scallops and saute for 1 minute on each side. Transfer to sauce for just a minute and season mixture with fresh lemon juice and chervil.
Make the sauce: In a saute pan set over moderately high flame, heat the oil and butter until hot. Add the shallots and cook, stirring, 1 minute. Add the mushrooms, thyme and salt and pepper and cook, stirring occasionally, for 3 to 5 minutes, or until mushrooms are soft. Add the garlic and cook, stirring, 1 minute. Add the wine and reduce by half. Add the demiglace and simmer 5 minutes. Add the cream and reduce until lightly thickened. Correct seasoning. Cover with a round of buttered waxed paper and keep hot.