Start the gravlax at least two days in advance (the sauce can also be prepared ahead of time, leaving just quick assembly on the day of the party). What to drink: Champagne (or sparkling wine) is a classic aperitif, and versatile enough to pair nicely with the salmon's slightly sweet mustard sauce as well as with the rich Foie Gras and Cream Eggs.
Heat all peppercorns and coriander seeds in small skillet over medium-high heat until spices are fragrant and seeds jump slightly, shaking skillet frequently, about 2 minutes. Crush spices in mortar with pestle or transfer to work surface, cover with kitchen towel (not terry cloth), and crush with mallet or bottom of heavy pan. Transfer spices to small bowl. Mix in salt and sugar.
Using small sharp knife, poke 12 small holes through skin of salmon. Rub 1/3 of spice mixture over skin. Sprinkle 1/3 cup chopped dill in bottom of 7x7x2-inch or 8x8x2-inch glass baking dish. Place salmon, skin side down, atop dill. Rub remaining spice mixture into top of salmon. Press 2/3 cup chopped dill onto salmon. Cover with plastic wrap, pressing directly onto fish. Place plate or another pan atop plastic. Place heavy cans on plate. Refrigerate 2 to 3 days.
Whisk mustard and vinegar in small bowl to blend. Gradually whisk in oil. Stir in chopped dill and salt. Season with ground black pepper. (Sauce can be made 3 days ahead. Cover and chill.)
Scrape spices and dill off both sides of salmon (some spice mixture will remain). Using knife with long thin blade, thinly slice salmon diagonally at 45-degree angle from top of fillet toward skin.
Arrange toasts on platter; top with salmon slices, sauce, and dill sprigs.