Season the chicken with sea salt and freshly ground black pepper. Heat the oil in a large dutch oven on medium high heat. Working in batches, sear the chicken, turning once or twice, until lightly browned, about 5 minutes. Transfer to a plate. Pour off the fat from the pot and discard.
Reduce the heat to medium, place the pan on the heat and melt 2 Tbsp butter. Add the onion and sauté until soft and golden, about 5 minutes. Add the carrots, potatoes, celery root, and thyme. Stir well to coat with the butter for a minute or two. Add the porter, GoBio! Organic Chicken broth, brown sugar, mustard and tomato paste to the pot along with the chicken thighs. Bring to a simmer and cover.
Reduce the heat to medium-low and simmer, stirring occasionally, for 30 minutes.
In a heatproof bowl, mash together the remaining 5 Tbsp butter and the flour to form a thick paste. Gradually whisk about 2 cups (500 ml) of the hot cooking liquid into the flour-butter mixture, and then stir this mixture into the pot. Cover and simmer, stirring occasionally, until the chicken is fully cooked, about 10 minutes.
Prepare the GoBio! Organic Gravy according to package instructions. Ladle the gravy over the chicken, serve immediately, with remaining gravy on the side.