Combine the spinach and red onions in a large bowl.
Heat the olive oil in a medium saute pan over medium-high heat until hot and shimmering. Add the mushrooms and salt to taste, and cook, stirring with a wooden spoon, until they start to release their moisture and begin to shrink, 2 to 3 minutes. Now let the mushrooms cook undisturbed until their liquid evaporates and they have turned deep golden brown on one side, 5 to 7 minutes. Stir and continue cooking until nicely browned on both sides, 3 to 5 minutes more. With a slotted spoon, remove the mushrooms from the pan and toss with the spinach and onions.
In a separate, heavy-bottomed saucepan, cook the pancetta over medium-low heat until dark and crispy, about 15 minutes. Remove from the pan with a slotted spoon and reserve.
To the rendered fat in the pan, add the vinegar, honey and mustard. Bring to a boil and whisk to combine. Add salt and pepper to taste and then pour the warm dressing over the spinach, mushrooms and onions. Add the reserved pancetta and toss to distribute the heat and dressing.