In a large pot over medium heat, pan-fry the bacon until their fat has rendered and the bacon is crispy. Remove the bacon with a slotted spoon and crumble the bacon when they are cool enough to handle. Do not discard the bacon fat.
Add the butter and olive oil into the pot with the bacon fat. When the butter has melted and the oil is hot, sear the chicken drumsticks over medium-high heat until all sides are golden-brown in colour, about 8 to 10 minutes. Do this in batches if the pot is too small, adding more oil if necessary. Remove the chicken onto a plate and set aside.
Reduce the heat to low. Add in the onion slices and crushed garlic and let them sweat till the onion is translucent, about 5 minutes. Add in the red wine, bay leaf, thyme, bacon bits and chicken drumsticks. Turn up the heat and bring the mixture to a boil. Cover, reduce the heat and simmer for 30 minutes, turning the chicken drumsticks several times during cooking.
Add the mushrooms into the pot, cover and simmer for another 15 minutes. Check that the chicken drumsticks are cooked through. Remove the chicken drumsticks and place them on serving plates and keep warm. Discard the bay leaf and thyme. Bring the sauce to a fast boil and reduce it by 1/3.
Mash the butter and plain flour together and add it into the sauce, stirring constantly until the sauce has thickened. Pour the sauce over the chicken. Garnish with parsley and serve immediately.