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Preheat oven to 400°F (~200°C)
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Cut onion in thick, moon-shape slices. Separate garlic gloves but do not peel. Mix onions and garlic cloves with 1 tbs of olive oil, sprinkle with salt, cover with foil.
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Cut cauliflower into bite-size pieces, add 2 tbs of olive oil, and salt and mix well. Spread on a baking sheet and cover with foil
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Cut portobello mushrooms into 1 cm thick slices, and arrange on a roasting pan. Brush both sides with 2 tbs olive oil, sprinkle with salt and black pepper. Cover with foil.
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Put all veggies in the oven. After 20 minutes remove foil, and bake the mushrooms and onions 5-10 minutes more until they are well browned. Leave cauliflower in the oven about 10 more minutes until it's nicely caramelized.
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Mix all veggies in a serving bowl, add more salt and pepper, drizzle with truffle oil, and a bit of balsamic vinegar, and sprinkle with the chopped hazelnuts.