Slice eggplant in half lengthwise. Place in open pan of boiling water, bottom side down, with water coming to about 3/4 of the way up on the eggplant. Boil until eggplant is just tender, about 15 minutes.
Meanwhile, brown Italian sausage, garlic and onion in olive oil till browned. Add bread crumbs, parmesan cheese, leaving burner on low.
Remove eggplant from the water and core out the eggplant, leaving about 1/2 inch around the edge.
Add additional eggplant to the sausage mixture and cook an additional 2-3 minutes. Add 1/2 cup marinara sauce.
Stuff eggplants with sausage/eggplant mixture. Top with remaining marinara sauce and mozzarella cheese.
Bake 25-30 minutes at 350 degrees until hot and bubbly.