Blend about one-third of the vegetables with 2 tablespoons of the butter or vegan margarine and enough of the reserved water to make a puree in a food processor or blender. Add this puree to the rest of the vegetables. Season to taste with salt and pepper. Spoon the mixture into a shallow ovenproof casserole. To make the crumble topping, put the oats into a bowl with all the nuts, the onion, garlic, herbs and salt and pepper. Add the rest of the butter or vegan margarine, and mix with a fork until the topping mixture resembles coarse breadcrumbs. Sprinkle the crumble mixture evenly over the top of the vegetables. Bake for 30-40 minutes, until topping is crisp and lightly browned.