Preheat the oven to 350 degrees F.
Place the onions in a large saucepan and cover with water. Bring to a boil and cook until the onions are partially tender, about 10 to 15 minutes. Drain and let cool.
When the onions are cool, cut each onion in half crosswise. Scoop out the center of each onion half, leaving a 1/2-inch shell. Reserve the centers. If necessary, cut a small piece from the end of each onion shell so the shells will stand upright.
Prepare a shallow baking dish large enough to hold the onion halves in one layer with non-stick pan spray. Place the onions in the prepared dish, hollowed sides up.
Chop the reserved centers of the onions. Saute in oil with the garlic in a medium saucepan until tender, about 5 minutes. Stir in the spinach, lemon juice, and pepper; cook until the liquid evaporates. Remove from the heat; stir in the bread crumbs and feta cheese
Fill the cooked onion shells with the spinach mixture. Cover with aluminum foil and bake approximately 25 minutes. Serve hot.
* Look for sweet onions that are light golden-brown in color, with a shiny tissue-thin skin and firm, tight, dry necks. (Ordinary storage onions are darker and have a thicker skin.) When cut into, sweet onions should have a creamy white interior. Avoid onions that have soft spots, surface bruises, or sprouting. Young onions are sweeter than old ones. They should have absolutely NO SMELL whatever. If they do, they are probably bruised somewhere under the skin and are on their way out.
Sweet onions are often named by geographic origin and described as being sweet. The best known are:
Vidalia from Georgia, Walla Walla from Washington, Maui from Hawaii, Imperial from California, Carzalia from New Mexico, The Texas Spring or Supersweet from Texas, and OSO Sweets from Chile, South America.
Baked Stuffed Onion with Spinach Feta Recipe: https://whatscookingamerica.net/Vegetables/StuffedOnion.htm