Rinse and drain the daal until the water runs clear. Set aside.
If pre-soaking, add the daal to a small bowl and cover with a few inches of water. Allow to soak for 8-12 hours
Melt 2 Tbsp clarified butter in a large stockpot.
Add the onions, ½ tsp salt and stir to coat.
Cook the onions over medium heat for ~10 minutes, until they've softened and just starting to caramelize. (Keep an eye on them, so they don't brown too much, or they'll color the dish)
Add garlic, the daal, 3 cups broth and the cheese rind, if using.
Stir well and make sure that the broth is covering the lentils by at least half an inch. If not, add a bit more.
Bring to a simmer, stir, and cover.
Over the next half an hour or so (the cooking time will depend on whether or not you were able to presoak the daal), stirring the pot every few minutes (this helps release the starch and add creaminess). As it cooks, watch the level of the broth, adding more as needed to just cover the daal.
After about 20 minutes, start to prepare the mushrooms.
Melt the other 3 Tbsp of clarified butter in a large skillet and add the mushrooms.
Turn the heat up to high
Stir lightly to coat them and allow them to cook without touching them (!) for at least three or four minutes.
Flip and repeat until they're golden on all sides.
Add just a little bit more clarified butter, garlic, & thyme leaves to the pan. Cook for another 1-2 minutes until they're fragrant
Season with a strong pinch of sea salt freshly ground black pepper, and a splash of balsamic vinegar, if desired.
The daal is done when they are tender, but not mushy.
(The consistency of the dish should be very much like a classic risotto: more solid than a soup, but thinner than a stew, so make sure that there is enough stock in the pot)
Remove the cheese rind.
Divide the cooked daal between the plates, garnish with shredded cheese a little more freshly ground black pepper.
Place the mushrooms on top and garnish with a sprig of thyme