In a large nonstick skillet melt 3 tablespoons of butter. Add the broccoli, mushrooms, and shallots. Cook for 5 minutes or until tender, stirring frequently.
Remove the broccoli mixture from the skillet. Add the remaining butter to the skillet and heat until melted. Stir in the flour, salt, and pepper. Cook and stir until bubbly. Gradually stir in the milk. Cook and stir until the mixture boils.
With a wire whisk, beat the egg yolk into the sour cream. Gradually mix the sour cream mixture into the sauce and cook until it thickens. Stir the broccoli mixture and turkey into the sauce.
Cook until hot. Serve over cooked pasta, biscuits or toast points.