Preheat the oven to 375°F (190°C).
If any of your mushrooms are dirty, gently brush them off with a damp towel. Remove and roughly chop the stems if they’re clean.
In a medium pan, heat 1½ teaspoons (8 ml) of the oil over medium-high heat and sauté the onion until soft and translucent, 5 to 7 minutes. Add the garlic and sauté for 30 seconds to 1 minute more, or until fragrant. Transfer to a large bowl.
Add another 1½ teaspoons (8 ml) of the oil to the pan along with the chopped mushroom stems. Leave them to fry and brown on the first side for about 2 minutes, then give them a stir and continue to fry. Once they’re browned all over, transfer them to the bowl with the onion.
Drain the artichoke hearts and roughly chop them. Add them to the bowl along with the nutritional yeast and basil. Stir everything together and let cool for about 5 minutes before adding the mayonnaise or else it will melt.
Meanwhile, in a small bowl, combine the breadcrumbs with the remaining 2 teaspoons (10 ml) of olive oil. Mix and mash together well.
Lightly oil a baking pan and place the mushroom caps on it, stem end up. Sprinkle a bit of salt and pepper inside each cap. If you want, you can brush a bit of olive oil on the mushroom caps.
Stir the mayonnaise, salt and a generous grind of pepper into the artichoke mixture. Taste and adjust the seasoning if you think it’s necessary. Use a small spoon to fill the mushroom caps with the mixture, packing it in and mounding it on top. Top with a spoonful of the breadcrumbs and press them into the filling.
Bake for 12 to 15 minutes, depending on the size of the mushrooms, or until the breadcrumbs are golden brown and the filling is heated through. If they are slow to brown on top, pop them under the broiler for a few minutes, watching carefully. Serve warm.