Preheat the oven to 350 degrees F. Remove the mushroom stems and chop them. Scrape out the gills with a spoon.
Heat the oil in a medium-sized saucepan and gently sauté the red onion, mushroom stems, red pepper, zucchini, capers and oats until the oats are golden. Stir in the basil, tamari or soya sauce.
Line a baking sheet with parchment paper. Oil the mushroom caps and put them baking sheet along with the limes.
Spoon the vegetable mixture onto the mushrooms. Season with salt and pepper.
Bake for 15-20 mins, until the caps start to soften.