Using a heavy bottom pan, heat the olive oil over medium-high heat. Add in the bacon when the oil is hot and cook until crisp. Remove the bacon, leaving the rendered fat in the pan and set aside.
Increase the heat to high. Add in the chicken, skin side down. Cook for 3 minutes. Check for browning. If it looks nicely browned, flip and repeat on the other side. When the chicken is browned, remove the pan from the heat. Using tongs, add the chicken to the bottom of the slow cooker, skin side down.
Reduce the heat to medium-high and return the pan to the heat with the fat still in the pan. Add in the onions with a good pinch of kosher salt. Stir and as the onion releases its water, scrape up the browned bits from the bottom of the pan. Cook the onions, stirring occasionally, for 8-10 minutes. When cooked, use tongs to remove the onions from the pan, keeping as much fat in the pan as possible. Add the onions over the chicken in the crock pot.
Return the pan to the heat and add in the leeks with a pinch of salt. Cook for 5-6 minutes, stirring well. If they look dry, add in olive oil. When they have browned just a bit, add them to the crock pot.
The pan should look fairly dry now. This is good. Add in the mushrooms and NO salt. Cook them until they start to release some water, about 3 minutes. Add in a pinch of salt, some olive oil and the garlic. Cook, stirring consistently, until fragrant (approx 4 minutes or so). Add the white wine and reduce for 2 minutes. Add the mushrooms and garlic to the crock pot.
Add the beef stock, red pepper flakes and kosher salt to the chicken. Roughly dice the cooked bacon and add to the slow cooker. Cook for 3 hours at the high setting on your crock pot.