1. Preheat oven to 375°.
2. Combine squash and thyme in a medium bowl. Crush 1 garlic clove; add to squash mixture. Drizzle with 1 tablespoon oil; sprinkle with 1/4 teaspoon salt and 1/4 teaspoon pepper. Toss to coat. Arrange squash mixture in a single layer on a baking sheet; bake at 375° for 40 minutes or until squash is tender; stirring occasionally.
3. Prepare grill.
4. Place 1 garlic clove in a mini food processor; process until finely chopped. Add 2 tablespoons oil, celery leaves, parsley, juice, and 1/8 teaspoon salt; process until finely chopped.
5. Sprinkle 1/4 teaspoon salt and remaining 1/4 teaspoon pepper evenly over trout fillets. Brush fillets with 1 teaspoon oil. Place fillets, skin side down, on a grill rack; grill 5 minutes or until fish flakes easily when tested with a fork. Keep warm.
6. Slice remaining clove of garlic. Heat a large skillet over high heat; add remaining 2 teaspoons oil. Add sliced garlic, onion, and remaining 1/8 teaspoon salt; cook 2 minutes or until onions begin to brown. Add squash mixture; cook 2 minutes or until hot. Stir in pecans; cook 1 minute. Divide squash mixture evenly among 4 plates; top each with a trout fillet. Spoon 2 tablespoons pesto over each trout fillet.