This creamy, cheesy baked Brussels sprout gratin will convince even the pickiest of eaters to gobble down their veggies. Gruyère cheese adds richness and flavor, while the panko adds a little crunch on top.
Preheat oven to 375°F. Coat a 2-quart baking dish with cooking spray. Place Brussels sprouts and water in a microwaveable bowl. Cover and microwave on High until tender-crisp, 5 to 7 minutes, depending on the size of your sprouts. Drain well. Transfer to the prepared baking dish.
Meanwhile, combine garlic, thyme sprigs and 2 tablespoons oil in a medium saucepan; cook, undisturbed, over medium heat, until the mixture sizzles, 45 seconds to 1 minute. Add flour and stir to combine; cook, stirring constantly, for 1 minute. Gradually whisk in milk and bring to a simmer, whisking occasionally. Simmer, whisking often, until thickened, about 2 minutes. Remove from heat; remove and discard thyme sprigs. Stir in salt, pepper and ½ cup cheese. Pour the sauce over the Brussels sprouts in the baking dish. Sprinkle evenly with remaining ½ cup cheese.
Toss panko with the remaining 1 ½ teaspoons oil in a small bowl. Sprinkle evenly over the mixture in the baking dish. Bake until golden brown and bubbly, 20 to 25 minutes. Garnish with thyme leaves, if desired, and serve immediately.