Rinse the fish well inside and out and pat dry with paper towels. Lay the big-boy out on a large rimmed sheet pan; cut 4 gashes crosswise on both sides of the fish, slicing all the way down to the bone. Season the arctic char with a generous amount of salt and pepper, inside the cavity and all over the skin. Stuff the mixed herbs and fennel inside the body of the fish. Tuck a lemon slice and bay leaf inside each of the slats. Rub a decent amount of olive oil on all parts of the fish. Roast for 30 to 40 minutes, until the skin is crispy and the fish itself is flaky. Transfer to a serving platter and garnish with fennel fronds. Serve with Pistou sauce.
Combine all ingredients in a food processor and process until the garlic is finely chopped. Serve at room temperature with grilled or roasted fish.