Prepare the pasta according to package directions, omitting salt; drain.

Meanwhile, heat the oil in a large nonstick skillet over medium-high heat. Add the onion and cook, stirring occasionally, until softened, about 5 minutes. Add the water and garlic and cook, stirring, until the liquid is evaporated, about 1 minute.

Add half of the tomatoes and cook, stirring, over medium-high heat, until the tomatoes begin to soften slightly, about 1 minute. Add the pasta, remaining tomatoes, lemon zest, 2 tablespoons of the lemon juice, salt and red pepper flakes. Cook, stirring, until pasta is heated through and the flavors are blended, about 5 minutes; remove from the heat. Add the ricotta, tossing gently to coat well. Taste for seasoning and add the remaining 1 tablespoon of lemon juice if needed. Sprinkle with the chives, if using, and serve at once.