Beat the egg in a large mixing bowl. Add the ground turkey, breadcrumbs, 1 Tablespoons olive oil, parmesan, garlic, oregano, basil, salt, and pepper in a large bowl. Mix everything up just so the ingredients are combined - avoid overmixing. Roll into desired size balls - mine were slightly larger than a golfball. Some people prefer them larger, some prefer them smaller. Place all rolled meatballs on a large baking sheet.
Place onion slices in the bottom of the crockpot, topped with 1/2 can of crushed tomatoes.
Coat a large skillet with remaining 1/2 Tablespoon of olive oil and bring to medium-high heat. Lightly brown the meatballs (only 6-8 meatballs at a time) for about 1 minute on each side. Do not skip this step - read above for why. Layer the meatballs into the crockpot as you're browning each batch. Once all browned meatballs have been added to the crockpot, pour remaining can of tomatoes on top.
Cover and cook on low for 6-6.5 hours. Serve with pasta, as meatball subs, plain, etc. No matter how I serve them, I always top with a little chopped fresh basil. Store leftovers covered tightly in the refrigerator for up to 1 week.