Preheat the oven to 400 F (200 C) and line a baking tray with parchment paper.
To make homemade breadcrumbs, add 2-3 slices of toasted or dry bread to the food processor. Pulse to make crumbs. Transfer the breadcrumbs to a large plate and set aside.
Add the oats to your food processor and blend them until they break into powder.
Add the rest of the ingredients except the breadcrumbs. Pulse to combine the ingredients. Test the mixture by forming a ball. If it crumbles, pulse a few more times. Test again and add aquafaba (chickpea liquid) a tablespoon at a time, pulsing and testing until you can form a ball. Add more oats if the mixture is too wet and melts in your hand.
Divide the mixture into 6 even pieces and form balls.
Roll each ball in the breadcrumbs. Form cutlets rectangular cutlets about ½ an inch thick. Place them on the baking sheet.
Bake the cutlets in the center of the oven for 20 minutes until they brown on top. Flip them and bake for an additional 15 minutes until they are golden brown.
How to Make Mustard Sauce
In a small bowl or cup, combine 1 tablespoon cornstarch with ¼ cup of water. Set aside.
Heat a saucepan to medium heat and add the diced red onion. Saute for 1-2 minutes to soften the onions. If the onions begin to stick, add water or vegetable broth a tablespoon at a time.
Add the garlic, thyme, and mustard powder. Stir for 30 seconds.
Whisk in the prepared mustard, then add the soy sauce and vegetable broth.
Let the sauce simmer for 2 minutes, then add the cornstarch slurry.
Whisk or stir the sauce for a few minutes and simmer for another 5 minutes to thicken.