Bring a large pot of salted water to a boil. Add tortellini and cook until just al dente, following package instructions (usually 2–4 minutes). Reserve 1 cup pasta water, then drain.
In the empty pot, heat olive oil over medium. Add shallot, sauté 2 minutes until soft. Add garlic and red pepper flakes, sauté 1 minute more. Season with salt and pepper.
In a large mixing bowl, whisk together pesto, 1 tablespoon white balsamic vinegar, and cream if using. Slowly whisk in 1/4 cup reserved pasta water until emulsified.
Add cooked tortellini, sautéed shallot/garlic, lemon zest, lemon juice, and half the parmesan to the bowl. Toss gently to combine. Add more pasta water if needed for a silky sauce.
Taste and adjust seasoning with salt, pepper, extra pesto, vinegar, or lemon as desired.
Serve warm or at room temperature, garnished with cherry tomatoes, basil, pine nuts, and remaining parmesan.