Fit a grilling grate over the direct heat of glowing embers. Combine the mustard, dill, brown sugar, garlic, paprika, salt, and pepper in a small bowl; spread the mixture evenly over the flesh side of the salmon. Place the salmon on a large piece of aluminum foil, skin side down; loosely wrap, seal tightly.
Place on the grilling grate directly over the heat source, and cook until desired degree of doneness, 20 to 25 minutes. Let stand 5 minutes. Serve with the lemon slice, if desired.