Line rimmed baking sheet with double layer of paper towels. Spread ricotta in even layer over towels; set aside and let sit for 10 minutes.

Place flour, Parmesan, panko, salt, pepper, and lemon zest in large bowl and stir to combine. Process spinach in food processor until finely chopped, about 30 seconds, scraping down sides of bowl as needed. Transfer spinach to bowl with flour mixture. Grasp paper towels and fold ricotta in half; peel back towels. Rotate sheet 90 degrees and repeat folding and peeling 2 more times to consolidate ricotta into smaller mass. Using paper towels as sling, transfer ricotta to bowl with spinach mixture. Discard paper towels but do not wash sheet. Add egg whites to bowl and mix gently until well combined.

Transfer heaping teaspoons of dough to now-empty sheet (you should have 45 to 50 portions). Using your dry hands, gently roll each portion into 1-inch ball.

Bring 1 quart water to boil in Dutch oven. Add 1½ teaspoons salt. Using spider skimmer or slotted spoon, transfer all gnudi to water. Return water to gentle simmer. Cook, adjusting heat to maintain gentle simmer, for 5 minutes, starting timer once water has returned to simmer (to confirm doneness, cut 1 dumpling in half; center should be firm).

Remove from the pot with spider.