• In a skillet, heat vegan butter over medium heat. Add the sliced onions and cook slowly, stirring occasionally, until deep golden brown and sweet (about 20 minutes).
• Place the peeled and chopped potatoes in a large pot and cover them with cold water by about 1 inch (2.5 cm). Add 1 teaspoon salt. Bring to a boil over medium-high heat, then reduce heat and simmer for 15–20 minutes, or until the potatoes are fork-tender.
• Drain the potatoes well and return them to the warm pot. Let them sit uncovered for 1–2 minutes to allow excess moisture to evaporate.
• Add vegan butter and mash until smooth.
• Gradually pour in the plant-based milk while mashing until creamy and fluffy. Adjust milk for desired consistency.
• Stir in black pepper, more salt to taste, and the caramelized onions. Cover and keep warm.
• Cook the lasagna noodles in salted boiling water according to package directions. Drain and set aside.

Assemble the casserole:

• Preheat oven to 375°F (190°C).
• Lightly grease a 9×13 inch (23×33 cm) baking dish with melted vegan butter.
• Spread a layer of lasagna noodles across the bottom.
• Add a layer of mashed potatoes, then sprinkle with vegan cheddar.
• Repeat the layers, finishing with mashed potatoes topped with the remaining cheese.
• Cover with foil and bake for 20 minutes. Remove the foil and bake for another 10 minutes, until the cheese is melted and bubbly.
• Garnish with chopped green onions. Toast breadcrumbs in vegan butter and sprinkle over the top for crunch.