Portobello Bacon Burgers with Celery Apple Slaw

Burger season is finally upon us! Since grass-fed beef is so lean, I like to incorporate other ingredients to give it flavor and moisture. This combination of portobello mushrooms, onions, and bacon lend a fabulous flavor. The original recipe does not include bacon, so you can leave it out, if you'd like. I used beef but feel free to substitute buffalo or another ground meat. Although many of these steps can be done simultaneously, you can also prepare the vegetables and bacon earlier in the day, or even the night before, and then finish the burgers just before your meal.

Adapted from epicurious.com.

Makes 4 large burgers.

Ingredients
Burgers
1 medium onion, peeled and cut into large chunks
12 oz portobello mushroom caps (about 4 large), cleaned, stems trimmed, and cut into large chunks
4 thick slices of nitrate-free bacon (optional)
2 Tbsp plus 2 tsp bacon fat or olive oil
1 lb ground beef or buffalo
freshly ground black pepper
sea salt (optional)

Slaw
2 celery ribs
1/2 Granny Smith apple, cored but left unpeeled
1 1/2 tsp cider vinegar
1 tsp olive oil
1 tsp whole grain mustard
1/4 tsp raw organic honey (optional)

Preparation

Bacon racked for baking.

1. Place bacon on a rack in a shallow pan (jelly roll pan works great). Set oven heat to 400 degrees. Immediately place bacon in oven (before it reaches temperature) until it is about halfway cooked, about 10-15 minutes. Remove from oven and cool slightly. You can also just cook it in a skillet, but baking works best for me.
   NOTE: If you don't have a rack, form crumpled pieces of foil into long strips and create a grid in your pan. Put the bacon on top of the foil grid. Not quite as pretty, but it works. You can also just set the bacon directly on a foil-lined shallow pan. Be sure to pat the bacon grease off the bacon strips before processing it.
2. While bacon cooks, working in batches, pulse mushrooms and onion in food processor until finely chopped. Remove mixture from processor.
3. Pulse partially cooked bacon in food processor until finely chopped.
4. Heat 2 Tbsp bacon fat or olive oil in a large heavy skillet. Add mushroom mixture and bacon, stirring to combine. Add 1/4 tsp pepper.
5. Cook, stirring occasionally, until vegetables begin to brown and liquid evaporates, 15 - 20 minutes (10 - 15 if not using bacon). Transfer to a large bowl and cool slightly, about 15 minutes.
6. While vegetables cool, prepare slaw by cutting celery and apple into 2-inch long julienne with a slicer (or by hand). Whisk together remaining ingredients and toss with apples and celery. Let stand 15 minutes (while burgers cook) to develop flavors.
7. Mix ground meat into cooled mushroom mixture with your hands until well combined. I know, I know...no one wants that stuff under their fingernails, but it's really the best way to mix. Form into four 4-inch patties.
8. Wipe out skillet and heat remaining bacon fat or oil. Cook burgers, turning once, for about 8 minutes total for medium rare. I think these burgers would also stand up to outdoor grilling, but I haven't tried it.
9. Serve topped with celery apple slaw.