Preheat oven to 400°. Butter ramekins or baking dish. Bring large pot of water w/ 1 t salt to a boil. Add pasta & cook according to directions. Drain well.
Meanwhile, in a mixing bowl, beat egg whites w/ electric mixer to soft peaks. In Dutch oven, melt butter over med-low heat. Add onion & cook until softened. Sprinkle flour over onion, stir constantly for 2 min, not allowing to brown. Add 1 C milk a little at a time, stirring constantly until smooth.
In an 8” square of cheesecloth, place bay leaf, thyme & peppercorns. Gather corners & tie w/ 100% cotton string. Add to Dutch oven with 1-2 t salt and remaining 2 ½ C milk. Increase heat to med-high and bring to boil, stirring frequently. Boil 1 min; reduce heat to low so barely simmering. Cook, stirring often, for 10 min more. Remove from heat, let stand for 5 min.
Remove herb bundle and immediately stir in ground black pepper, nutmeg, and the cheeses. Stir until cheeses are melted. Stir cooked macaroni into cheese mixture. Fold in beaten egg whites. Stir in approximately 2 T White Truffle Oil.
Spoon into baking dish(es). Top w/ Panko (garlic) bread crumbs. Bake 15 min or until heated through and crumbs are toasted. (Make garlic bread crumbs by sautéing 3 cloves minced garlic in 3 T butter; mix w/ bread crumbs.)
To serve, drizzle lightly w/ white truffle oil and sprinkle w/ thyme.