Heat a a saucepan over medium heat. When hot, add the olive oil and the minced onion. Saute until the onions are translucent, 3 minutes.
Add the mushrooms and cook until they release their moisture and begin to brown.
When the stock is boiling, add the reserved roux and stir well to combine. Add this mixture to the onions and mushrooms.
Add the sour cream and again stir well to combine.
Add the fresh basil, reduce heat and simmer for 5 minutes.
Add the light cream and continue simmering until the sauce begins to thicken. If you want to spice up this dish to give it some pizzazz, now's the time to add a few drops of hot sauce. You could also have some at the table to serve those who like it hot.
Add the shrimp and cook until the shrimp are pink about three minutes.
Season with salt and pepper and serve over rice.