Remove the reconstituted porcinis from their liquid, reserve the liquid, and roughly chop. Set aside.
In a 3 or 4-quart Dutch oven or sauce pot, melt two tablespoons of the butter.
Once the butter is melted, add half of the cremini mushrooms and half of the shiitake mushrooms. (They will soak up the butter immediately, then begin to release their liquid. They will absorb the liquid and begin to brown. Funny little guys, aren't they?)
Allow the mushrooms to brown, stirring every 2 minutes, until both sides are nicely browned.
Transfer the mushrooms to a plate or bowl. Set aside.
Add the remaining two tablespoons of the butter and repeat the cooking process with the remaining cremini and shiitakes.
Turn the heat down to medium-low and add the shallot, garlic, garlic and onion powders, porcinis, and previously cooked mushrooms.
Cook, stirring often to soften the garlic and shallot, about 3 minutes more. It should be very aromatic.
Add the stock, reserved porcini liquid, one teaspoon of kosher salt, and 1/2 teaspoon of the white pepper. Stir to combine, taste, and add more white pepper if desired.
Add the half and half and stir well. Adjust the seasoning to taste.
If you like the consistency of this soup, you can serve as is. If you would prefer a thicker, creamy soup, mix the flour and water until dissolved and add to the soup.
Stir and allow the soup to come up to a gentle boil, stirring often, until thickened.
Serve the Cream of Mushroom Soup very warm.
Serve with my Homemade Croutons, if desired.