Prepare your grill for high, direct heat.
While the grill is heating, sauté the baby spinach in a tablespoon of olive oil and a sprinkle of salt in a large sauté pan on medium high heat, just until the spinach wilts, then remove to a plate.
Wipe the mushrooms clean with a damp paper towel. If you are working with whole mushrooms, slice off the stems. Paint the tops of the mushroom caps with some of the oil from the sun-dried tomatoes.
Scrape the grill grates clean. Using a folded up paper towel dipped in olive oil, wipe down the grill grates to oil them. Brush the onion rounds with some olive oil and place them on the grill. Place the mushroom caps on the grill, top side down first. Sprinkle with a little salt. Grill the mushroom caps and the onion rounds for 4-5 minutes per side. Grill the burger buns, cut side down, for about 1/2 to 1 minute, until lightly toasted.
To assemble, spread some sauce on each bun. Put a portobello mushroom cap on the bottom, then layer on the spinach, sun-dried tomatoes, and finally some onion. Serve immediately.
Adapted from: www.simplyrecipes.com