To make the glaze, whisk the ketchup, mustard, brown sugar, apple cider vinegar, and salt in a small bowl. Set aside.
Meatless Loaf
Preheat oven to 375 °F (190 °C). Prepare a loaf pan by lining with parchment paper. This makes it much easier to remove after cooking. Set aside.
Combine the water and flaxseed meal in a small bowl, and set aside for at least 10 minutes until it thickens.
In a large pan, heat oil over medium heat. Add onion, celery and a sprinkle of salt. Sauté for 5-6 minutes, until onions are lightly golden and celery is softened.
Add garlic, thyme, rosemary, parsley, and sage. Sauté for 30-60 seconds until fragrant.
Turn the heat to medium-high, then add the mushrooms (and a little more oil if the pan looks dry). Spread them out and let them sit mostly undisturbed for 2–3 minutes so they release their liquid and start to brown—don’t stir too often. Once the liquid evaporates, give them a quick toss and keep cooking until nicely golden, about 2-3 more minutes.
Transfer the cooked mushroom mixture to a large bowl. Add the mashed potatoes, breadcrumbs, flax eggs, rice, rolled oats, walnuts, tomato paste, Worcestershire sauce, salt, and pepper. Combine all ingredients really well, using hands if necessary. Taste to see if it needs any more seasoning.
Place mixture into your lined loaf pan. Press down firmly so it molds into pan. Brush the top of loaf with the ketchup glaze.
Bake for 30-40 minutes, until heated through. If you would like to crisp the glaze a bit, turn the broiler on for 2-3 minutes. Be careful not to burn.
Let the loaf cool for about 10 minutes before removing from the pan. Use the paper to pull it up and onto your serving dish. Let it sit for another 10 minutes before slicing it. This helps firm it up more.
Notes
Cleaning Mushrooms: Use a damp paper towel or a soft brush to clean each mushroom. Or, you can lightly rinse the mushrooms with water, but don't soak them. Pat dry when done. Mushrooms are like sponges and will absorb a lot of water.I pulse the mushrooms a few times in a food processor to get them finely chopped and make life easier. Be careful not to over process or they will be mushy. If you have a small processor, you may need to chop in two batches. If you only have whole flaxseed, you can grind the seeds in a coffee grinder.Make the rice mashed potatoes the night before and store them in the fridge. This will cut your prep time down.