1. Heat a large skillet over low heat. Once the pan is warm, add about 1 tablespoon oil and the mushrooms.
2. Cook mushrooms for about 10 minutes, stirring occasionally, until tender and cooked through.
3. Sprinkle over ½ teaspoon Old Bay (or similar seafood seasoning) and cook 1 minute longer. Transfer mushrooms to a large mixing bowl and let cool.
4. Using the same pan, increase heat to medium and add a little more oil and the onions. Sprinkle lightly with salt and cook for about 5 minutes, stirring occasionally, until translucent.
5. Turn off heat, add onions to the bowl with mushrooms, and let cool slightly for 5–10 minutes.
6. Add remaining seafood seasoning, breadcrumbs, vegan mayonnaise, parsley, and a pinch of salt and pepper. Stir gently to combine and adjust seasoning to taste.
7. Add Just Egg and stir gently to combine. Cover and chill for at least 1 hour.
8. Using a ¼ cup measure, portion mixture into 7–8 crab cakes and place on a lined baking sheet. Shape gently and avoid flattening too much.
9. Pan-fry option: heat a skillet over medium-low to medium heat. Add vegan butter or oil, arrange crab cakes 1–2 inches apart, and cook 3–5 minutes per side until lightly browned.
10. Oven-bake option: preheat oven to 400°F. Brush tops lightly with oil and bake 15 minutes, flipping halfway through, until lightly golden brown. Serve and enjoy.