Drain ricotta in a sieve for at least 15 minutes (or up to 2 hours if watery).
Wilt spinach in boiling water for 1–2 minutes, then transfer to ice water.
Squeeze out all moisture from spinach and finely chop.
In a bowl, mix ricotta, spinach, egg, parmesan, breadcrumbs, lemon zest, nutmeg, and salt.
Add flour in two batches and gently mix until combined.
Dust a tray with semolina flour. Scoop and portion dough into 20 pieces.
Roll into balls and dust lightly with semolina.
Chill for at least 1 hour (up to 12 hours).
Bring salted water to a boil. Test one gnudi first.
Cook gnudi until they float, then boil for 1 extra minute.
Remove and let rest briefly.
For sauce, melt butter with Calabrian chilies over medium heat.
Add pasta water gradually and whisk into a smooth sauce.
Toss cooked Spinach Ricotta Gnudi gently in the sauce and serve immediately.