In a frying pan over a medium heat warm 2 tablespoons of the oil then add the onion and cook while stirring until soft and beginning to turn golden, about 25 minutes. Add sugar and continue cooking, stirring as necessary, until soft and dark brown. Should not be at all crispy. Lower the heat if the onions are crisping.
In another frying pan, cook chanterelles in the remaining olive oil over medium heat until they are soft and golden, about 15 to 20 minutes.
Slice the baguette into 2cm slices, and arrange in a single layer on a baking tray. Bake in the preheated oven until just toasted.
Remove from the toast from the oven and arrange a small amount of onions on top then a few mushrooms slices on top of the onions. Serve warm.