Preheat the oven to 375 degrees Fahrenheit.
Mix the water and flaxseed meal together in a small bowl, set aside for at least 10 minutes (this makes 2 flax eggs). It will gel up.
Prepare a loaf pan by lining with parchment paper. Lining the pan makes it so much easier to remove after cooking. Here is a quick video on how to line a loaf pan with parchment paper. Set aside.
In a large pan, heat the oil over medium heat. Sprinkle with salt. Sauté onion & celery over medium heat for 5-6 minutes, until onions are slightly browned and celery is softened.
Add garlic, thyme, rosemary, parsley, and sage. Sauté for 1 minute.
Turn heat up to medium-high and then add the mushrooms. Cook until they have released all their liquid and the pan and mushrooms are mostly dry. About 6-8 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 & pepper. Combine all ingredients really well, using hands if necessary. Give a quick taste to see if it needs any more seasoning, add as needed.
Place mixture into your lined loaf pan. Press down firmly so it molds into pan. Brush 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 trying to remove from pan. Use the paper to pull it up and onto your serving dish. Let sit for another 5-10 minutes before serving. This helps firm it up more.