Bring a large pot of salted water to a boil. Add the pasta and cook according to package directions, until al dente.
Just before draining, remove 1 cup of the pasta cooking water. Drain.
Meanwhile, while the pasta cooks, make the pasta sauce.
Heat the oil in a large skillet over medium heat. Add the onion and the jalapeño pepper (or Serrano) and sauté until the onion becomes translucent, about 3 minutes.
Stir in the garlic and baby spinach and cook tossing with kitchen tongs until the spinach starts to wilt, about 2 minutes. Remove from the heat.
Transfer the spinach mixture to a blender. Add the roasted peppers, cilantro, sour cream, chicken bouillon and 2/3 cups of milk or half and half and process until smooth and creamy. Add more milk, if needed.
Transfer the poblano pepper mixture to the same pot you used to cook the pasta. Heat over medium-low heat for about 2-3 minutes, stirring as needed.
Add half of the cream cheese and stir until it melts. Repeat with the remaining cream cheese. Lower the heat and simmer for about 4 minutes. Season to taste with salt and black pepper.
Add the pasta and toss to combine. Add some of the reserved pasta cooking water if needed to make the sauce creamy. Serve garnished with chopped cilantro, crumbled cheese and pepitas, if desired.