Heat the oven to 375 degrees. Wash the pike inside out and dry it on paper towels. Cut 2 or 3 diagonal slashes in both sides of the fish so the oven heat can penetrate and cook the fish more evenly. Tuck herb sprigs into each slash. Trim the tail into a neat v shape. Set the fish in an oiled baking dish or roasting pan and sprinkle it with olive oil, salt and pepper. Roast the fish, brushing regularly with more olive oil, until the skin is brown and crisp, 30 to 40 minutes. The fish eye will have turned white and the flesh should no longer be transparent; check it by flaking a bit from the backbone with a fork. Herb Butter: Bring a large saucepan of water to a boil. Discard the stems from the large watercress and spinach and the large stalks from the parsley and chervil. Add the herbs to the boiling water and blanch them by simmering for 2 minutes. Drain, rinse with cold water and pat dry on paper towels. Drain the anchovies and work them in a food processor with the pickle, capers garlic and butter until finely chopped. With the blades turning, slowly pour in the olive oil. The herb butter will be creamy and a lovely shade of green. Add the mustard and season the butter to taste with lemon juice, salt and pepper. To finish, transfer the whole fish to a warm platter and decorate it with lemon wedges. Serve the herb butter in a separate bowl.