Sauté the onion with vegan butter (margarine) or some oil in a skillet/pan. After a few minutes, add garlic, cauliflower, and carrots. Pour in the vegetable broth and let it all simmer for a further 8-10 minutes with a closed lid at low heat.
In the meantime, peel the potatoes and cut them into pieces, then add them to a pot with boiling salted water. Cook the potatoes over medium heat for about 10-15 minutes or until they are fork-tender. Drain the potato water, then add the potatoes back into the pot. Add the plant-based cream/milk and the spices. Mash it all together with a potato masher, until it becomes a potato puree (don't use a food processor or a blender, otherwise the puree will turn out sticky).
After 8-10 minutes, add the thawed peas and the corn to the pan. Next, stir in the spices, mustard, and lemon juice.
Mix the plant-based cream and cornstarch in a bowl using a whisk. Pour this mixture into the pan and stir. It thickens up after a short time.
Taste and adjust seasonings. Feel free to add more salt, pepper, thyme, lemon juice or mustard. If the sauce is too thick, add a splash of plant-based cream/milk. If it's too thin, simply add more cornstarch or flour (mixed with plant-based milk or cream).
Serve and enjoy the creamed vegetables with mashed potatoes. Store leftovers in an airtight container for up to 3 days in the fridge or freeze for up to three month.