In a glass or ceramic bowl, combine the tomatillos, green onions, cilantro, jalapeno, salt, pepper, and lime juice. Toss to mix, cover with plastic wrap and chill for at least 30 minutes and up to 4 hours so that the flavors can marry. Return the salsa to room temperature before using.
Dig the marrow out of the bones with a small sharp knife, scraping against the bone to get all of it, or push it out with your fingers if it will come out in one piece. Dip the knife into very hot water and cut the marrow into 1/4inch dice, dipping again as necessary. Refrigerate the marrow until you are ready to use it.
Pour oil to a depth of 1/2-inch in a small skillet. Fry the tortillas on both sides until crisp and golden. Drain on paper towels and keep warm in a low oven.
Bring a large skillet of lightly salted water to a slow simmer. Remove from the heat and immediately plunge all of the marrow into the hot water. Leave in the water for 1 minute, then use a slotted spoon to transfer it to a warm plate, draining well.
Place a warm tostada on a warm plate and top with some of the salsa. Top each tostada with a spoonful of the warm marrow and scatter a few grains of the sea salt over the marrow. Add a few cubes of avocado, season with pepper to taste and serve immediately.
c.1997, M.S. Milliken & S. Feniger, all rights reserved