How to Make It
Preheat oven to 425°F. Toss together carrots, oil, 3/4 teaspoon salt, and pepper on a baking sheet lined with parchment paper. Roast in preheated oven 20 minutes. Flip carrots; add lemon halves, cut sides up, and garlic to pan. Roast at 425°F until carrots are fork-tender and browned in spots and garlic is soft, 15 to 25 minutes. Let cool 10 minutes.
Squeeze baked lemon halves into a small bowl to yield 1 tablespoon juice, and set juice aside; discard lemon halves. Remove and discard garlic skins; smash garlic cloves into a paste using flat side of a chef’s knife. Combine lemon juice, garlic paste, egg yolks, and remaining 1/2 teaspoon salt in a blender. With blender running, gradually drizzle in hot melted butter until mixture is thickened, about 1 minute and 30 seconds. Add additional salt and pepper to taste; process about 5 seconds, adding up to 4 teaspoons water, 1 teaspoon at a time, if needed to achieve a pourable consistency.
To serve, divide carrots evenly among 4 plates; drizzle with hollandaise sauce, and sprinkle with pistachios.