Preheat the oven to 400 degrees F. Line a large baking sheet with parchment paper.
Remove the leaves and trim the top and bottom of each kohlrabi bulb. Cut into ½" round slices. Place on the parchment paper (do not overlap) and brush each side with olive oil. Sprinkle with salt and pepper. Bake for 30-35 minutes, or until they are fork tender.
About 10-15 minutes into the roasting time, prepare the sauce. Heat the olive oil and butter in a skillet over medium heat. Add the sliced mushrooms, thyme, and rosemary. Cook for 5 minutes, or until the mushrooms are tender.
Add the vegetable broth to the skillet and bring to a boil. Reduce heat to a low boil/simmer and let the sauce "reduce" so that it becomes thicker, about 5-10 minutes.
Transfer the roasted kohlrabi from the baking sheet to serving plates, and top with spoonfuls of the mushroom sauce. Enjoy warm!
This is a great vegan main dish, but it's fairly low in protein. Serve with a bean or nut-based side, or a grain like wild rice or quinoa.