-
1
Place mushrooms in a medium bowl and rinse with cold water, shaking them in the water once or twice.
-
2
Using a slotted spoon, transfer mushrooms to a small pot, leaving any grit at the bottom of the bowl behind, and add enough water to cover, about 1 1/2 cups.
-
3
Set over low heat and bring to a simmer (this can take up to 45 minutes).
-
4
Remove from heat and immediately lift mushrooms out of the liquid using a slotted spoon.
-
5
Chop mushrooms and set aside.
-
6
Strain mushroom broth through a coffee filter or cheesecloth into a medium bowl.
-
7
Decant broth again by pouring it into another bowl and leaving any grit at the bottom behind.
-
8
Set broth aside.
-
9
Place parsley, garlic, and onion in a food processor and pulse into a coarse chop, about 10 pulses.
-
10
Set aside.
-
11
Place a heavy 4-quart pot fitted with a lid over medium-low heat.
-
12
Add oil and butter.
-
13
When butter foams, add onion-parsley mixture.
-
14
Cook until onions are translucent, about 6 minutes.
-
15
Stir in chopped mushrooms, then stir in tomato paste until incorporated.
-
16
Slowly stir in reserved mushroom broth.
-
17
Fill one of the tomato paste cans with cold water and add it to the mixture.
-
18
Repeat 4 or 5 more times (for a total of about 2 cups of water), stirring with each addition.
-
19
Season with a big pinch each of salt and freshly ground black pepper.
-
20
Cover and simmer over the lowest possible heat for 60 to 75 minutes, adding more liquid if the sauce becomes too thick and stirring from time to time.
-
21
Taste and season again with salt and pepper as needed.
