Season the chicken thighs with the 2 tsp. salt and the 1/2 tsp. pepper. In a Dutch oven or other heavy pot over medium-high heat, warm the oil until very hot but not smoking. Working in batches, add the chicken thighs, turning once or twice, until lightly browned on both sides, about 5 minutes per batch. Transfer to a platter or push to the sides of the pot. Pour off the fat in the pot.
Reduce the heat to medium, add 2 Tbs. of the butter to the pot and let it melt. Add the onion and sauté until golden, about 6 minutes.
Add the carrots and potatoes.
Stir in the porter, broth, brown sugar, mustard, tomato paste and thyme.
Return the chicken thighs to the pot, submerging them in the liquid, and bring to a simmer. Cover, reduce the heat to medium-low and simmer, stirring occasionally, for 30 minutes.
Cut the carrots and potatoes into 1-inch chunks. Peel and chop the onion.
Mash together the remaining butter and the flour to form a thick paste. Gradually whisk this mixture into the pot.