Heat the oil in a large skillet over medium-high heat
Add the onions and cook, stirring, until starting to soften.
Stir in 1/2 cup water & bring to a boil.
Reduce heat to medium-low and simmer until most of the liquid has disappeared.
Stir in another 1/2 cup water, and continue cooking 30-40 minutes, adding more water when needed.
Stir in maple syrup and balsamic vinegar.
Cook, stirring until onions are brown and most of the liquid has evaporated.
Remove from heat and set aside
Wash mushrooms and remove stems.
Put mushrooms in a shallow dish and whisk together olive oil, vinegar, salt, and pepper.
Pour olive oil mixture over mushrooms, turning to coat
Allow mushrooms to sit for 30 minutes while onions are cooking.
Oil grill with cooking spray and heat to medium-high.
Grill mushrooms covered, about 5 minutes each side
During the last minute of grilling, top each mushroom, stem side with cheese. Cover and heat until the cheese is melted. Remove from grill.
While mushrooms are grilling assemble the slider buns by spreading each with Dijon mustard and mayonnaise on each of the halves.
Top the bottom halves with lettuce, bacon, and caramelized onions.
Add the cheese-topped mushrooms, and top with the other half of the slider bun.
This recipe makes a one pint jar of Maple Caramelized Onions, and you will only use a portion of it for these sliders, so you'll have extra on hand to store in the fridge and use as a delicious condiment.Many of the steps in the recipe are simultaneous, so you can do a lot of the prep work while the onions are caramelizing and the mushrooms are grilling.