Bring 4 quarts water to boil in a large pot for cooking pasta.
Place ricotta, Parmesan, butter, 1/4 teaspoon salt, and 1/2 teaspoon pepper in bowl large enough to hold cooked pasta.
Fry oil and bacon in medium nonstick skillet over medium heat until crisp, 6 to 7 minutes. Transfer bacon to plate lined with paper towels. Add onion to empty pan and cook until lightly golden, about 1 minute. Transfer onion mixture to bowl with ricotta mixture.
Meanwhile, add 1 tablespoon salt and pasta to boiling water. When pasta is about 1 minute shy of al dente, add peas and continue to cook for 1 minute. Reserving 1 cup cooking water, drain pasta and peas. Add 1/2 cup cooking water and lemon juice to ricotta mixture and whisk until smooth. Add pasta and peas to bowl and toss to coat, adding more reserved cooking water as necessary to moisten pasta. Stir in crisp bacon and adjust seasonings with salt and pepper to taste. Serve.