Peel the small potatoes and cut in half, then coat with oil. Score the tomatoes crosswise. Cut the lemon into slices, then cut each slice in half.
Wash the cod and pat dry. Season with 3 tablespoon lemon juice, salt, and pepper.
Wash the herbs if needed and pat dry, then place half of each herb variety in the cod's belly opening. Place the cod diagonally in an oven pan and arrange the potatoes, tomatoes, and half the lemon wedges around the cod. Season the potatoes and tomatoes with salt and pepper, and drizzle 100 ml olive oil over everything.
Bake in oven at 200°C (400°F) on the 2nd rack from bottom for 20 minutes.
While the fish cooks, finely chop the remaining herbs and mix with 25 ml olive oil. After 20 minutes, drizzle the herb mixture on the potatoes and tomatoes, sprinkle sea salt on top, and continue baking for 10 minutes.