Begin by removing the stems from 4 large Portobello mushrooms, ensuring the caps are clean and intact. Rinse gently under cold water if necessary, then pat dry with a paper towel.

In a medium bowl, combine ¼ cup soy sauce, 1 tablespoon paprika, 1 tablespoon maple syrup, 1 tablespoon light brown sugar, 2 tablespoons tomato paste, 1 teaspoon liquid smoke, and 1 teaspoon oregano. Whisk these ingredients together until smooth.

Brush the marinade generously over the Portobello caps, ensuring they are fully coated. Place the marinated mushrooms in a shallow dish and let them soak for 15 minutes, turning them halfway through.

While the mushrooms are marinating, heat 2 tablespoons of olive oil in a frying pan over medium heat. Add one chopped onion and sauté for about 10-12 minutes, stirring frequently until the onions soften and begin to caramelize.

Once the onions are caramelized, stir in 1 tablespoon of soy sauce and 1 tablespoon of maple syrup. Continue to sauté for an additional 3-5 minutes until the onions become sticky and deeply flavorful.

Preheat a grill pan over medium heat and brush it lightly with grapeseed oil. Place the marinated Portobello mushrooms on the grill and cook for about 3 minutes on each side, brushing them with leftover marinade.

Remove the grilled Portobello mushrooms and place them on soft vegan hamburger rolls. Top each burger with caramelized onions, fresh lettuce, sliced tomatoes, dill pickles, and creamy avocado.

Serve the assembled Grilled Vegan Portobello Burgers immediately while hot, alongside your favorite sides.