In a skillet over medium-high heat, saute onion in olive oil. When onions are soft, add spinach and smoosh down with lid to cover. Cook until spinach is wilted and cooked through (about 5 minutes).
Drain the onion-spinach mixture in a sieve, then finely chop. Place chopped mixture in a medium sized bowl and add ricotta and Parmesan cheese. Season with salt and pepper and stir to thoroughly combine. Taste to check seasoning.
On a floured or marble board, divide the pizza dough into 4 to 6 pieces (depending on how big or small you want your bastelle). Form each piece into a thinly stretched circle and place heaping spoonfuls of the spinach mixture evenly among pieces. Fold the dough over to make a half-circle and crimp the edges to seal.
Place the pieces on a greased sheet pan, and brush the tops with a tiny bit of olive oil. Cook for 15 minutes, or until pizza dough is golden brown and crusty to the touch. Allow to cool slightly before serving, as filling is quite hot.