In a pan large enough to hold the mushrooms in a single layer, heat the butter over med high until sizzling. Add the mushrooms with a pinch of salt and pepper and cook stirring constantly until they are deep brown and have carmelized.
Add the shallots and the garlic, and just enough water to cover the mushrooms. Add the parsley and scrape the brown bits. Transfer the vegetables and broth to a smaller pot.
Bring to a boil over med high heat and boil for 15 minutes. Strain broth with a fine mesh strainer. It will be brown in color.
Reduce over high heat by cooking 10 minutes longer until just a syrup remains. About 1/2 cup. It will resemble a veal demi glace. Serve alone as an intensely flavored sauce.
For milder flavor, swirl in 2 to 3 tbsp olive or mushroom broth. Note this will give a broken sauce effect creating a gold and brown sauce.