Preheat the oven to the 180 degrees C, air circulation and elements (not grill), rack in the middle.
In a large rectangular non-stick oven dish add the fish, insert in the belly the herbs, place the lemon slices all over the fish, drizzle with the oil and season with the salt & pepper.
Cover well with parchment paper and tightly close with aluminum foil. Cook for 35 minutes.
In the meantime, put a small saucepan on medium heat and melt the butter. Once it foams toss the capers and sauté for some 30 seconds, pour the lemon juice, bring to a simmer, reduce the heat and stir continuously until the sauce has thickened a bit, reduced by about 1/3, for about 3 minutes. Season with salt and pepper.