Preheat oven to 180C/350F.
In a large baking dish, toss the onion, oil, balsamic glaze, salt & pepper. Cover with foil & bake for 1 hour & 10-15mins.
Slice the tip of the garlic to expose the cloves then drizzle with oil. Wrap in foil then roast for 1 hour, or until deep golden, sticky & soft.
Cook the pasta until al dente. Drain, reserving a cup of the pasta water.
Squeeze the roasted garlic into the caramelised onions, mix in the sun-dried tomatoes, mascarpone, and a splash of pasta water. Stir in the cooked pasta, then mix in parmesan & basil, using more pasta water to thin out if needed.
Season with salt & pepper to taste, then serve with more parmesan if desired.
