In a small sauce pan add mushrooms and 1 ½ cups stock, bring to a low simmer. Reduce to low and let simmer for 20 minutes until mushrooms are tender. Add a bit more stock as it evaporates – you want to end up with at least ¾ cup of liquid.
After the mushrooms are tender, drain liquid (reserve) and chop mushrooms up. If desired, strain the liquid through a cheesecloth to remove any grit, and set aside.
In a large skillet, sauté onions, garlic, and thyme with oil until soft. Add the tomato paste and stir for about 1 minute, then add wine. Simmer until reduced by half.
Add in mushrooms and liquid, and stir in butter. Season to taste with salt and pepper.
Just before serving, stir in the lemon juice and parsley. Toss with cooked pasta and top with cheese if desired.