Roast the garlic by peeling and wrapping it in tin foil with a splash of veggie broth. Roast at 400 F (205 C), in a regular or toaster oven, while you cook the veggies.
Put potatoes (peel if you don't want specks of skin), yellow pepper, carrots, onion and cashews in a pot and cover with water. If you don't have a high speed blender and are grinding the cashews, then don't boil but grind them dry and place them in the blender.
Cook over high heat until water boils, about 15 minutes if uncovered. Turn heat to medium and boil another 20 minutes, until potatoes are tender.
When done, drain water (keeping some to thin out sauce if prefer a thinner sauce), separate veggies and put the cashews in the blender. I use a fork to fish out the veggies, then drain the cashews over a pasta strainer.
Measure out the needed amount of each veggie and place each in the blender. Add the rest of the ingredients.
Blend on high until you get a creamy gooey consistency. If you have a tamper tool use it to help mix the sauce as it blends. If you don’t have one, then you will need to stop and push down the ingredients every once in a while.
Taste and adjust ground mustard seed for a sharper flavor if desired.
If you want to thin it out, add some of the reserved cooking water, 1 tablespoon at a time until you get the desired consistency. Use it while it's nice and hot!