In a medium saucepan, melt the butter over medium. Add the mushrooms and cook, stirring, until slightly browned, 5 to 6 minutes. Add the shallot and garlic, season with salt and pepper and stir until softened, 2 to 3 minutes. Stir in the flour until well incorporated. Stir in the sherry, and then the cream. When the sauce bubbles, reduce the heat to low and simmer until slightly thickened, about 1 minute. Add the nutmeg and season. Stir in the tarragon.
In a large skillet, heat the EVOO, one turn of the pan, over medium-high. When the oil ripples and starts to smoke, add the shrimp and toss until they're just opaque in the centers, about 4 minutes. Season, stir in the lemon juice and add the shrimp mixture to the pan with the mushroom sauce.
Place the toast points on dinner plates. Top with the shrimp and mushroom sauce. Garnish with the parsley.