Sauté the onions in 1 tablespoon olive oil over low-medium heat until they begin to brown. I like to keep them covered and stir every couple of minutes.
While the onions are cooking, mix the remaining ingredients and let sit (so the psyllium expands to thicken the batter).
Once the onions are cooked, remove from heat and let cool slightly.
Preheat oven to 325F (163C).
Pour batter into a parchment-lined glass baking dish, 8"x11.5" (200mmx290mm) or slightly smaller, and spread evenly.
Spread sautéed onions on top.
Bake at 325F (163C) for about 22 minutes, or until batter is cooked through.
Cut into servings and enjoy!
Notes
A note about substitutions: You could substitute unflavored pea protein for the rice protein (or a pea protein blend, like Warrior Blend). I wouldn't use whey or casein as they would give a different (undesirable) texture.