Preheat the oven to 400 degrees F (200 degrees C). Grease a small casserole dish. Brush portobello mushrooms on both sides with olive oil and place in the casserole dish.

Heat 1 tablespoon olive oil in a skillet over medium-low heat and cook onion until soft and translucent, about 5 minutes. Add garlic and cook until fragrant, about 30 seconds. Stir in dates and rosemary and cook until soft, about 5 minutes. Drizzle with maple syrup and balsamic vinegar and cook until mixture caramelizes, 3 to 5 minutes. Season with salt. Top portobello mushrooms evenly with the stuffing mixture.

Bake in the preheated oven until mushrooms are soft and cooked through, 10 to 15 minutes.