Preheat oven to 400°; cut onion into wedges; toss onions with 1 tbsp oil + salt/pep; roast 15-17 min until onions are browned

Line a second sheet pan with foil; drizzle fish 2-4 tbsp pesto, 1 tbsp oil + salt/pep; bake 8-12 min until firm, opaque + cooked through

Meanwhile, boil a large pot of unsalted water + gently loosen linguine with hands; carefully place in water, stir immediately + boil 2-3 min until al dente; drain + return to pot

Toss linguine with onions + 2-4 tbsp pesto; divide onto plates, top with filets + enjoy!