Set these onto a parchment lined baking sheet. Bake in a 400F oven on the centre rack till 165F is reached in the interior of the meatballs. An instant read thermometer is your friend here, you don't want to overcook chicken meatballs of any kind, or else they will dry out. This should take between 20-25 minutes depending on your oven. If you want a bit more colour, just place under the broiler for a few minutes, keeping your eye on them. These can be baked off ahead of time and stored in the fridge till ready to bake with the sauce and cheese. Bring back to room temperature before proceeding
Add the tomato paste, Italian seasoning. thyme and basil and stir through to coat the onion. Pour in the puréed tomatoes. Stir and bring to a gentle bubble. Add the butter and bay leaf. Reduce the heat and let it simmer and thicken with the lid only slightly on. Or use a splatter guard. After 20 minutes, stir in the parmesan cheese, if using. After another 15 minutes, stir through the salt, pepper and chilli flakes. Taste and re-season if desired. This can be stored in a sealed container in the fridge till needed.
Preheat the oven to 425F. Have the baked meatballs at room temperature if you pre-baked them in advance. Warm the sauce to bubbling on the stove top before the next step.
Spray a baking dish or skillet with non stick spray. Pour in about 2 cups of the marinara sauce. Scatter 1 cup of the thinly sliced baby spinach on top and stir through.
Place the baking dish into the oven and bake till the cheese is bubbling and browned, about 15 minutes. Finish under the broiler for extra melty browned goodness.
Drain.
Count on 2 oz of pasta per person. If you made the meatballs the larger size, 3 or maybe 4 will be enough, depending on who you are feeding!
Leftovers are easily stored in a container and reheated in covered skillet or 300F oven.
Depending on how much everyone likes sauce, you can keep the rest of the sauce warm ready to serve. If not, store in a sealed container and use in soup, stew or with leftover meatballs.
Smaller meatballs will take less time in the oven. Or you can just brown them in a skillet and finish cooking them in soup or sauce as desired.