Preheat the oven to 400°F (200°C).
In a large bowl, toss the baby potatoes, baby carrots, and green beans with 1 tablespoon olive oil, 1/4 teaspoon salt, and 1/8 teaspoon black pepper.
Spread the vegetables evenly on a large baking sheet and roast in the oven for 15 minutes.
While the vegetables roast, place the white fish fillets on a separate baking dish lined with parchment paper.
In a small bowl, combine the remaining 1 tablespoon olive oil, minced garlic, dried thyme, dried parsley, 1/4 teaspoon salt, and 1/8 teaspoon black pepper.
Brush the olive oil and herb mixture evenly over the fish fillets.
Top each fillet with 2-3 lemon slices.
After the vegetables have roasted for 15 minutes, place the baking dish with the fish in the oven alongside the vegetables.
Bake both the fish and vegetables for an additional 15 minutes, or until the fish flakes easily with a fork and the vegetables are tender.
Remove from the oven and place the roasted vegetables in a serving dish.
Add 1 tablespoon unsalted butter to the hot vegetables and toss to coat.
Sprinkle the chopped fresh parsley over the fish and vegetables before serving.