Preheat the oven to 450F/232C. Wash the potato(es) and poke with a fork on all sides. Wrap up in tin foil tightly, and place in the oven directly on the wire rack. Bake for approximately 50 minutes. This will depend on the size.
Wash and peel the beet(s). Steam for about 12-15 minutes, until fork tender, but not too soft. (A good indicator is being able to lift the beet off with a fork without it falling off the fork. If it falls off the fork, it may be slightly overdone. But it will still be okay to eat, just softer.)
Wash, pat dry with a towel and slice the mushrooms. Preheat a non-stick frying pan on medium heat, toss in the chopped mushrooms, and sprinkle with a bit of salt and pepper. (Salt will draw out the moisture, so you do not need to add oil or water to the pan. Just monitor the heat closely.) Sauté until the mushrooms are golden on both sides, slightly softer, but not mushy.
Wash, peel and slice the carrot. Steam a few minutes. Wash and chop the broccoli and add it to the carrots to steam for a few minutes as well. The carrots are done when you can pierce with a fork and lift it off. Broccoli is ready when vibrant green.
If using raw corn, boil/steam it or cook it accordingly. If using canned, simply rinse and set aside. If using frozen corn, microwave or rinse under water to thaw.
Rinse the chickpeas, and black beans.
Slice the avocado and set aside. Discard the pit and skin. (Or skip this step, and slice on the day of eating this meal prep meal instead to preserve freshness.)
Assemble the prepared components mentioned above portioned out in bowls or sealable containers, and top with cashews (optional). Drizzle with lemon juice before serving.