Bring a large pot of salted water to a boil and cook the whole grain spaghetti according to package directions until al dente.
While pasta cooks, roast the poblano peppers over an open flame or under a broiler until the skin is charred and blistered.
Place roasted peppers in a bowl and cover for 5 minutes, then peel off the charred skin, remove the seeds, and roughly chop.
Season the chicken breast with half of the sea salt and black pepper, then grill in a pan over medium-high heat until cooked through.
In a small skillet, sauté the chopped onion and garlic in olive oil until softened and fragrant.
Place the roasted poblanos, sautéed onion, garlic, Greek yogurt, cilantro, lime juice, and remaining salt and pepper into a blender.
Blend the sauce until completely smooth and vibrant green.
Slice the grilled chicken into strips.
Toss the cooked spaghetti with the creamy poblano sauce and top with the sliced chicken breast.