Trim top and bottom of fennel bulb, quarter lengthwise, remove tough core from each slice, and cut into ¼" slices. Place on prepared baking sheet and toss with 2 tsp. olive oil, ¼ tsp. salt, and ¼ tsp. pepper. Spread into a single layer and roast until lightly browned, 10-12 minutes. Remove from oven. Fennel will finish cooking in a later step. While fennel roasts, prepare ingredients.
Prepare the Ingredients
Trim zucchini ends, halve lengthwise, and cut on an angle into ½" slices. Halve grape tomatoes. Thinly slice garlic. Stem and mince parsley. Halve lemon lengthwise. Cut one half into four wedges and juice the other half. Rinse trout, pat dry, and season flesh side with ¼ tsp. salt and a pinch of pepper.
Make the Sauce
Place a medium non-stick pan over medium heat. Place 1 Tbsp. olive oil and garlic in hot pan and cook, stirring occasionally, until garlic is tender, 1-2 minutes. Remove from burner and stir in 2 tsp. lemon juice, mustard, and parsley (reserve a pinch for garnish). Season with ¼ tsp. salt and ¼ tsp. pepper. Remove to a bowl, wipe pan clean, and reserve.
Roast the Vegetables
Carefully, add tomatoes and zucchini to baking sheet with fennel. Toss with 1 tsp. olive oil, ¼ tsp. salt, and ¼ tsp. pepper and spread into a single layer. Roast until zucchini is tender, 9-11 minutes. While vegetables roast, cook trout.
Cook the Fish
Return pan used to cook sauce to medium-high heat. Add 2 tsp. olive oil and trout, skin side up, to hot pan. Cook until lightly browned, 2-3 minutes. Flip, reduce heat to medium, and cook until skin is crispy and trout reaches a minimum internal temperature of 145 degrees, 3-5 minutes.
Plate the Dish
Place vegetables on plate and garnish with remaining parsley. Place trout next vegetables and sauce on top of trout. Garnish with lemon wedges.