Serve these individual casseroles with a tossed green salad and toasted rounds of French bread.
1. Preheat the oven to 350° and generously butter 6 ramekins. Melt the 3 tablespoons butter in a small saucepan. Sprinkle on the flour, mustard, and salt. Stir over medium heat until the mixture foams. Remove from the heat and whisk in the milk. Return to medium heat and stir until thickened. Remove from the heat and blend in the cayenne, tarragon, and Parmesan cheese.
2. Spoon a small amount of sauce into each ramekin. Divide the avocados, shrimp, and eggs among the ramekins. Spoon on the remaining sauce, then sprinkle on the buttered bread crumbs. Bake, uncovered, for 25 to 30 minutes or until bubbly and nicely browned.