Preheat oven to 350 degrees F. Drain and crumble tofu and reserve. Place whole onion(s) in boiling water for 10-12 minutes. While onions are boiling, microwave spinach in a covered microwave proof dish for 4 to 5 minutes (Note: since all microwaves are different, check after 3 minutes and continue for 1 minute more if necessary); let cool and drain. Remove onion(s) from pot with a slotted spoon; allow to cool, then peel and cut in half crosswise. Scoop out center of onion leaving a 1/2" shell and reserving center(s). Place shell(s) in a shallow baking dish. Chop center of onion(s) and mince garlic; heat oil in a skillet on medium and sauté onion and garlic until tender, about 3-4 minutes. Reduce heat to medium-low and stir in spinach, crumbled tofu, lemon juice and pepper. Cook until liquid evaporates, about 3-4 minutes. Remove from heat and stir in cheese and breadcrumbs, reserving 1-2 teaspoons of breadcrumbs. Spoon spinach mixture into onion shell(s) and sprinkle with reserved breadcrumbs. Cover shells with foil, bake about 20-25 minutes and serve.