To make the glaze, whisk the ketchup, mustard, brown sugar, apple cider vinegar, and salt in a small bowl. Set aside.

Preheat oven to 375 ?F (190 ?C) and prepare a loaf pan by lining with parchment paper.

Combine the water and flaxseed meal in a small bowl; 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; saut? for 5-6 minutes until onions are lightly golden and celery is softened.

Add garlic, thyme, rosemary, parsley, and sage; saut? for 30-60 seconds until fragrant.

Increase heat to medium-high and add mushrooms, spreading them out. Let them sit mostly undisturbed for 2-3 minutes until they release their liquid and start to brown. Cook until nicely golden, about 2-3 more minutes.

Transfer the cooked mushroom mixture to a large bowl. Add mashed potatoes, breadcrumbs, flax eggs, rice, rolled oats, walnuts, tomato paste, Worcestershire sauce, salt, and pepper. Combine all ingredients well.

Place mixture into the lined loaf pan and press down firmly. Brush the top of the loaf with the ketchup glaze.

Bake for 30-40 minutes until heated through. If desired, turn on the broiler for 2-3 minutes to crisp the glaze, being careful not to burn.

Let the loaf cool for about 10 minutes before removing it from the pan using the parchment paper. Let it sit for another 10 minutes before slicing.