Turn the Instant Pot onto the sauté function and add the butter. Let melt.
Add the mushrooms and garlic, and cook, stirring frequently, until mushrooms are nicely browned. Add the thyme and sprinkle lightly with salt and pepper. Cook another 30 seconds. Turn off the Instant Pot
Place the cut up chicken just on top of the mushrooms and season with more salt and pepper. Add the broth. Secure the lid and set to manual function (high) to cook for 5 minutes.
Once the cooking is done, let the pressure release naturally for 5 minutes, then vent it to relieve the remaining pressure.
Use a slotted spoon to scoop out the chicken onto a plate, leaving the mushrooms in the pot. Add the cream and mustard, whisk to combine and set on saute again. Cook a few minutes until the sauce has thickened a little.
Remove about half a cup of the liquid and whisk in the glucomannan. Add back to the pot and stir in to make a thicker, richer gravy. Add the chicken back into the pot and toss to combine.
To cook this recipe on a stovetop, melt the butter over medium heat in a dutch oven. Sauté the mushrooms and garlic with the thyme, salt, and pepper as directed. Stir in ⅔ cup broth (you need additional liquid for the stovetop) and bring to a simmer, then stir in the chicken. Cover and simmer until the chicken is cooked through, about 15 to 20 minutes. Transfer the chicken and mushrooms to a bowl while you make the sauce (you don't need to remove all of the mushrooms but you do want all the chicken pieces out so they don't get overcooked). Add the cream and mustard, and cook until it thickens, then whisk in the glucomannan or xanthan gum. Add the chicken back in and toss to combine.