Root Vegetable Gratin

Source

via Gfycat
Source: [Sunset]( https://www.sunset.com/recipe/root-vegetable-gratin-4)
1/2 pound medium red potatoes, scrubbed but not peeled
1/2 pound medium red beets, peeled
6 ounces yellow beets, peeled
7 ounces fennel bulb, green stalks trimmed (save one big feathery frond for garnish)
1/4 pound round red radishes
2 tablespoons unsalted butter
2 tablespoons minced fresh rosemary
2 tablespoons minced fresh thyme, divided
1 shallot, finely diced
3 garlic cloves, grated
1 1/2 cups whipping cream
1 tablespoon flour
About 2 tsp. each fine salt and freshly ground pepper
3/4 cup finely shredded Gruyère cheese
1/4 cup finely shredded parmesan cheese
Using a handheld slicer, slice potatoes, beets, fennel, and radishes into 1/8-in.-thick slices. Preheat oven to 325°.
Put butter, shallots, and garlic in a small saucepan and cook over medium-low heat until aromatic, about 1 minute. In a small bowl, whisk 2 tbsp. flour with 1/4 cup cream until smooth. Whisk in remaining cream, then pour into saucepan along with 1 tbsp. thyme, the rosemary, salt, and pepper; let simmer until shallots are translucent, about 5 minutes.
Butter a 1- to 1 1/2-qt. ceramic or cast-iron gratin dish or shallow baking dish. Arrange half of potatoes, beets, radishes, and fennel slices tightly to cover bottom of dish, overlapping them like dominos and alternating vegetable types, in an oval around edges and a row up center. (You may have to tuck fennel pieces in since slices may not hold together.) Season lightly with salt and pepper. Pour half of cream mixture over slices, stirring as you pour so shallots and garlic are evenly distributed, and top with half of Gruyère. Arrange remaining sliced vegetables the same way, then top with remaining cream mixture and Gruyère. Sprinkle parmesan on top.
Cover gratin with oiled foil and bake until vegetables are tender when pierced with a fork, 1 to 1 1/2 hours. Remove the foil and broil until golden and bubbly, about 5 minutes.
Top with remaining 1 tbsp. thyme. Let sit 10 minutes before serving.
**Some notes:** I recommend adding a little cayenne and nutmeg in addition to the herbs they use here. If using a mandolin, please use the guard and exercise caution!
They whisk flour with cream here to get it incorporated, which is all well and good, but I usually just make a **roux** by adding the flour straight into the butter and cooking it for a bit, then whisking in warm cream slowly. However, either method will work! The end result is that the flour cooks and the starch thickens the liquid into a sauce.