Flaky Cod with Celery Salad

Ingredients: 4 cod fillets Salt and pepper to taste 2 tablespoons olive oil 1 tablespoon butter 1 lemon, sliced (for garnish) Celery Salad: 4-5 celery stalks, thinly sliced 1 small red onion, thinly sliced 1/4 cup fresh parsley, chopped 1/4 cup fresh dill, chopped