Toss mushroom caps and stems, olive oil, and salt and pepper to taste in medium bowl. Arrange caps, gill side down, in a single layer on a large low-sided roasting pan or jelly-roll pan with stems placed alongside. Roast until mushrooms have released some juice and are brown around edges facing the pan, 12 to 15 minutes. Remove pan from oven and turn caps over with a metal spatula. Continue to roast until mushroom liquid has completely evaporated and mushroom caps and stems are brown all over, about 5 to 10 minutes longer.
Once roasting pan is removed, readjust oven rack to center position and leave oven temperature at 450 degrees. Leave the cooked mushroom caps on roasting pan, but remove cooked stems and set aside.
Meanwhile, fry bacon in a large skillet over medium heat until crisp, 5 to 7 minutes. Remove and drain on paper towels; discard all but 1 tablespoon of drippings. Add onion; cook until soft, 6 to 8 minutes. Add garlic; cook about 1 minute longer. Add spinach; cook until wilted, about 2 minutes. Transfer spinach mixture to large bowl; let cool for 10 minutes.
Transfer spinach mixture to food processor, along with bread crumbs, ricotta, one ounce blue cheese, and roasted mushroom stems; process to a chunky puree, scraping down the sides of the processor at least once to ensure an even texture. Return spinach mixture to large mixing bowl; stir in bacon bits. Fill each mushroom cap with a heaping teaspoon of filling; top each with a fig half and a portion of remaining blue cheese.
Adjust oven rack to lowest position and heat oven to 450 degrees.
Roast stuffed mushrooms until cheese is melted and filling is hot throughout, about 8 minutes. Serve warm.