Bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally, until tender yet firm to the bite, about 11 minutes. Drain well.
Meanwhile, heat olive oil in a large skillet over medium heat. Add onion and cook until softened, about 2 minutes. Add frozen shrimp and Espelette pepper and lower heat to medium-low. Cook until shrimp is no longer frozen, 3 to 5 minutes.
Add drained spaghetti to the skillet along with feta cheese. Increase heat to medium-high. Toss to coat spaghetti in the mixture until feta has melted and become a sauce. Remove from the heat and toss in parsley, mint, and chives until incorporated. Divide between 2 plates.