Preheat the oven to 350°F. Lightly grease a 5x9-inch loaf pan or a 9x13-inch baking dish.
Toss the mushrooms with the carrots, onions, thyme and oil on a baking sheet. Roast for 10 minutes, until soft.
Combine the soy sauce, beef stock and buttermilk in a saucepan. Sprinkle gelatin over the liquid and let it 'bloom' for ten minutes. Once hydrated (it should have an applesauce-like texture), warm the mixture slowly over medium heat until the gelatin is just dissolved. Don't go too hot or the buttermilk will curdle.
In a large bowl, combine the gelatin mixture with the roasted vegetables and remaining meatloaf ingredients. Mix well. Transfer to the prepared loaf pan, or shape into a loaf and place in a 9x13 inch baking dish.
In a small bowl, combine all of the meatloaf glaze ingredients until smooth. Brush the glaze over the top of the meatloaf and bake for 1 hour, until cooked through.
In a large stockpot, place the potatoes and a generous pinch of salt. Cover with cold water and bring to a simmer. Cook at a simmer until the potatoes are fork-tender, about 15 minutes. Drain the potatoes and return to the pot. Add the butter, milk, buttermilk, salt and pepper. Mash until smooth. Fold in the horseradish. Season with salt and pepper to taste.
Bring a large pot of salted water to a boil. When it boils, submerge the peas all at once for one minute, until bright green and crisp-tender. Immediately remove from the water with a strainer and run under cold water.
In a small bowl, combine all of the herb-chile oil ingredients. Season to taste with salt and pepper.
To serve, slice the meatloaf and divide between plates with mashed potatoes and peas. Drizzle with chile oil.