Heat the olive oil in a large skillet over low heat. Add the onions and a pinch of salt. Have a measuring cup full of water nearby. Cook the onions, stirring frequently, until they begin to brown. Turn up the heat to medium and continue caramelizing the onions, allowing a glaze to develop on the bottom of the pan. Add a few tablespoons of water and scrape up the brown bits, incorporating them into the onions. The heat will cause the water to evaporate. Repeat this process until the onions have turned a dark brown color and are both bitter and sweet in flavor, approximately 20 minutes. Remove from the heat and set aside.

In the same pan (or a different pan if you’re cooking multiple dish components at once), cook the bacon over medium-low heat to render out the fat. Once the fat has mostly melted down, turn up the heat and brown the bacon until it's caramelized and crunchy, approximately 5 minutes. Set aside to drain on a paper towel. Remove all but two tablespoons of the bacon fat from the pan. Add the sliced shiitake mushrooms and cook over medium heat until soft but slightly chewy, approximately 3-5 minutes.

Preheat the oven to 450 degrees F. In a medium saucepan, bring the 2 cups of water to a boil and then add the steel cut oats and reduce the heat to a simmer. Cook until the liquid has completely absorbed, approximately 8-10 minutes (this will take less time than normal steel cut oats since it’s a smaller water ratio).

Lightly grease a 12-inch skillet or casserole dish. In a large bowl, stir together the cooked oats, caramelized onions, bacon, mushrooms, parsley, salt and pepper. Gently press flat into the prepared baking dish. Baked for 10-12 minutes, or until the top has browned slightly.