Preheat oven to 425ºF/220ºC. Place parchment paper on a large cookie sheet. Place potatoes in one corner of cookie sheet, drizzle with 2 tablespoons of olive oil, and sprinkle with salt. Place carrots and shallots in another corner of cookie sheet, drizzle with 2 tablespoons of olive oil, sprinkle with salt, pepper, and thyme. Place pan in oven for 5 minutes. Remove, stir, place back in oven for 5 minutes. Remove, stir (potatoes and vegetables separate), place back in oven for 5 minutes. Place fish on cookie sheet. Mix together mayonnaise, dijon, and garlic cloves. Spread over fish. Sprinkle nuts over the fish. Stir fennel in with shallots and carrots. Place back in oven for 5 minutes. Remove, stir (potatoes and vegetables separate), place back in oven for 5 minutes. Check that fish is cooked to 145ºF/65ºC.