Cook the pasta: Fill a large pot with water, add 2 tbsp salt, and bring to boil. Cook 1 lb spaghetti until al dente, about 10 minutes. Reserve 1 cup pasta water before draining.
Roast poblanos: Preheat broiler and arrange 4 poblanos on foil-lined baking sheet. Broil 8-10 minutes, flipping halfway through, until skin is blackened and blistered.
Steam peppers: Cover a baking sheet with foil and let poblanos steam for 5 minutes. Rub off and discard as much charred skin as possible.
Prepare poblanos: Transfer to a cutting board, slice open, and remove stems and seeds. Discard all charred skin, stems, and seeds from poblanos completely.
Blend sauce: Add roasted poblanos, 8 oz cream cheese, ¾ cup Mexican crema, ¼ onion, 2 garlic cloves, and 1 tsp bouillon to the blender. Blend until completely smooth and creamy.
Cook sauce: Melt 1½ tbsp butter in a large pot over medium heat. Pour in poblano cream sauce and cook for 3 minutes, stirring occasionally.
Combine pasta: Add cooked spaghetti to the pot and toss to coat with sauce. Cook 1 minute more, adding pasta water if needed.
Serve immediately: Remove from heat and serve in bowls topped with crumbled queso fresco and fresh cilantro leaves for perfect presentation.