Drain the ricotta in a fine mesh sieve for at least 15 minutes (up to 2 hours if watery).
Place spinach in a heat-proof bowl and cover with boiling water. Let wilt for 1–2 minutes, then transfer to ice water.
Squeeze out all excess water from the spinach and chop finely.
In a large bowl, combine spinach, ricotta, egg, parmesan, breadcrumbs, lemon zest, nutmeg, and salt. Mix gently.
Add flour in two batches and mix until just combined. Avoid overmixing.
Dust a tray with semolina flour. Scoop dough into portions and place on tray.
Roll into balls, dusting lightly with semolina flour.
Chill for at least 1 hour (or up to 6 hours for best texture).
Bring salted water to a boil. Test one gnudi first; it should float in about 4 minutes. Cook an additional minute.
Cook remaining gnudi using the same method.
For the sauce, melt butter with Calabrian chilies over medium heat. Add pasta water and swirl.
Gradually add remaining butter, alternating with splashes of pasta water to create a smooth sauce.
Add cooked Spinach & Ricotta Gnudi to the sauce and gently toss. Serve immediately.