Boil the Pasta and Shrimp: In a large pot of salted water, boil the spaghetti. In the last five minutes of the pasta boiling, add the shrimp. Reserve ¼ cup of pasta water prior to draining.
Make the Sauce:
On medium heat, in a stainless steel skillet lightly covered with olive oil, saute the onions and garlic until fragrant. Add the grape tomatoes.
Once the skins lightly burst, lightly press down on them. Add the chili flakes, lemon juice, and half of the reserved pasta water to the pan, and combine.