Heat oil in large nonstick skillet over medium-high heat. Add onions. Saute until pale golden, about 5 minutes. Reduce heat to medium; saute onions until tender and deep golden, about 15 min longer. Transfer 3/4 cup sauteed onions to small bowl; reserve. Add peas to onions in skillet. Saute until peas are crisp-tender, about 3 minutes. Remove skillet from heat.

Cook pasta in large pot of boiling salted water until just tender but still firm to bite. Drain pasta, reserving 1 cup cooking liquid.

Add pasta and 1/2 cup cooking liquid to onion mixture; stir over medium-high heat 30 seconds. Mix in ricotta, basil, and lemon peel, adding more cooking liquid to moisten as needed. Season with salt and pepper