When potatoes have cooled enough to handle, scoop out the flesh from potatoes and transfer to a large bowl. (Throw the skins away.) This will give you about 6 cups of cooked sweet potatoes.
Add about half of the warmed butter and herbs to the sweet potato mixture. Set aside.
Add cubed bread to the saucepan with the remaining herb/butter mixture. Toss to coat. Add in salt and pepper. Transfer to another small bowl (separate from potato mixture.)
Do not clean out the saucepan. Heat the other two tablespoons of vegan butter in the same saucepan. Add in chopped shallots and saute until shallots are tender and translucent and slightly browned -about 5 minutes.
Add in coconut milk, cornstarch, and nutritional yeast. Continue to cook for a couple of minutes or until bubbly and slightly thickened.
Add to the potato mixture and with an immersion blender, blend until mostly smooth. If you do not have an immersion blender, add the coconut cream and shallots to a blender and blend till smooth. Then add in about half of the potato mixture and continue to blend.
Pour blended potato mixture into the prepared 9x13 pan. Bake uncovered for 30 minutes.
Take out of the oven and sprinkle seasoned bread crumbs on top. Continue to bake uncovered for another 15-20 minutes or until breadcrumbs are crisp and lightly browned. Sprinkle lemon zest on top of baked casserole. Let cool slightly before serving.