Baked Pasta with Cauliflower and Swiss Chard

Preheat oven to 425 degrees. In a large, straight-sided skillet, melt half of butter over medium-high. Add garlic, chard stems, cauliflower, and sage. Season with salt and pepper. Cook, stirring, until garlic is golden, 2 minutes. Add 1/2 cup water; cover. Cook until cauliflower is just tender, 5 minutes. Transfer to a bowl; stir in chard leaves.

Cook pasta in a pot of salted boiling water until al dente. Drain, reserving 1 1/2 cups water. Add remaining butter and flour to skillet; cook over medium-high, whisking, until toasted, 1 minute. Slowly whisk in pasta water until smooth. Bring to a boil; cook, whisking, until mixture coats the back of a spoon, about 2 minutes. Reduce heat to low; whisk in sour cream and 1 1/2 cups cheese. Stir in pasta and chard mixture; top with remaining cheese. Transfer to oven; bake until cheese melts, about 10 minutes. Serve.