Preheat the oven.

In a bowl, mix ketchup, mustard, brown sugar, apple cider vinegar, and salt.

In another bowl, combine ground flaxseed and water; let it sit to thicken.

Heat avocado oil in a pan and sauté red onion, celery, and garlic until soft.

Add finely chopped mushrooms to the pan and cook until moisture evaporates.

In a large bowl, combine cooked rice, mashed potatoes, rolled oats, breadcrumbs, walnuts, tomato paste, thyme, parsley, sage, rosemary, vegan worcestershire sauce, and the flaxseed mixture.

Stir in the cooked vegetable mixture.

Season with sea salt and pepper to taste.

Transfer the mixture to a loaf pan and smooth the top.

Spread the ketchup mixture over the top.

Bake in the preheated oven for 25 minutes.

Allow to cool slightly before slicing.