Preheat your oven to 400 F (200 C).
Peel medium butternut squash and halve it length-ways. Scoop out the seeds, then cut the squash into ~1-inch cubes. Place on a rimmed baking tray, toss with 1 tbsp of olive oil, lemon juice and the chili flakes. Roast for 30 minutes.
Either place into bowl and mash with a potato masher / blend with an immersion blender, or puree in a blender. Mix in the maple syrup and apple cider vinegar and season with salt to taste.
Meanwhile, add the onions to a dry frying pan and cook, stirring over a medium heat until browned. Add the remaining oil and a pinch of salt, and continue to cook for about 5 minutes until softened. Stir the caramelised onions into the mashed squash.
Spread about 1 tbsp of ricotta onto a toasted slice of bread and top with a generous amount of squash mixture. Serve warm or room temperature.