In a heated pan, add oil. Add the onion and begin to caramelize, adding salt and pepper, until fragrant and turning golden brown. Remove from the pan.
Place the carrot, parsnip, and mushrooms into the pan, adding a little more oil and butter once they begin to entirely soak up the oils from the previous onions. Add the herbs and spices at this time, continue to mix until the vegetables begin to brown but still have a crunch.
If the potatoes aren't nearly done, remove this from the heat to maintain the crunch and as to not overcook.
To make the potatoes, keep the skin on. Cube and use a hand mixer or hand masher. Add a little butter, salt, and milk (and the optional ingredients, if you wish); continue to mash or whip, and add what you see fit.
Begin to make the stew from your vegetables. Add the onions and fresh chopped garlic to the vegetables and stir. Add broth or stock and tomato paste. Taste and season as you need.
Continue to stir and bring to a bubble. Reduce and whisk in the tablespoon of flour. It will thicken. Serve immediately over mashed potatoes. Chili flakes or thick sea salt give it a fun kick.