Heat a tablespoon or so of olive oil in a large 12'' saucepan or skillet (make sure it has a tight-fitting lid). Once hot, add the Italian sausage and cook, breaking up the meat every few minutes until browned and no longer pink.
Remove the cooked sausage from the pan and place onto a paper towel to drain. Set aside.
If needed, pour a bit more oil into the pan, then add the chopped onion. Sauté for a few minutes until soft and translucent, then add the garlic and cook for 1 minute more.
Mix in the tomato paste, Calabrian chilis, paprika, Italian seasoning, salt, and pepper until well combined. Cook for 1 minute, then carefully pour in the canned tomatoes, using a spoon or your hands to break them up into a sauce-like consistency.
Add the cooked Italian sausage back to the pan, stirring until well incorporated. Season the shakshuka with salt and pepper to taste and allow it to come to a simmer.
Once bubbling, use a spoon to make 6 to 7 wells into the sauce and place a slice of mozzarella into each well. Carefully crack an egg on top of each mozzarella slice.
Cover the saucepan with the lid, adjust the heat to low, and allow the eggs to cook for around 7-8 minutes, until the whites are set. The yolks should be cooked just under your liking, as the residual heat will continue to cook the eggs as it sits.
Remove the pan from the heat, garnish with fresh basil, and serve warm with toasted bread.