Preheat the oven to 400°F (200°C).
In a large bowl, toss the baby potatoes, baby carrots, and green beans with 1 tablespoon of olive oil, 1/2 teaspoon salt, and 1/4 teaspoon black pepper.
Spread the vegetables evenly on a large baking sheet and roast in the oven for 15 minutes.
While the vegetables begin roasting, pat the white fish fillets dry with paper towels.
In a small bowl, combine the remaining 1 tablespoon olive oil, minced garlic, dried thyme, dried parsley, 1/2 teaspoon salt, and 1/4 teaspoon black pepper.
Remove the baking sheet from the oven after 15 minutes and push the vegetables to the sides to create space in the center.
Place the fish fillets in the center of the baking sheet.
Brush the garlic and herb mixture evenly over the fish fillets.
Top each fillet with 2-3 thin lemon slices.
Return the baking sheet to the oven and bake for an additional 10 minutes, or until the fish flakes easily with a fork and the vegetables are tender.
Remove from the oven and serve the fish fillets alongside the roasted vegetables.