Preheat oven to 400F

1. Prepare the onions. Slice each onion in half, longways tip to root. Remove the outer paper-like skin then remove all but 3 or 4 layers from the inside of the onion, leaving a bowl shape. Repeat with the remaining onions. Save the inside for another purpose. 
2. In a deep baking dish (8 x13 or whatever size will hold all the onions, I used a cast-iron skillet b/c I was only baking 4 halves.) add 2 cups of water and place the onions in the water with the hollowed out part facing up, season generously with salt and pepper. Bake for 30 minutes.
3. Prepare the filling. In a deep skillet, begin by browning the sausage. When the sausage is halfway cooked add in the garlic and red pepper flake (if using) cook 2 minutes. Then add in the tomatoes and cook another 5-8 minutes season with pepper, lastly add in the diced mushrooms. Once everything is cooked down and sautéd, taste and season with salt and pepper if needed. Note – the sausage has a lot of seasoning so go easy on the salt until you have tasted it. Stir in the basil and cashews, if using.
4. Carefully remove the baking dish from the oven. Load up the sausage mixture into each onion. Top with shredded cheese. Bake another 15 – 30 minutes depending on how firm or soft you like the onion.