Start by preparing the white roux paste. In a small, but deep bowl, whisk the flour and half the softened butter vigorously until a thick creamy paste is formed. Set aside.
Stir the tomato paste into the vegetable broth until blended.
In a large skillet warmed to medium, melt the remaining butter and lightly sauté the onion and garlic. Only for about a minute or until aromatic.
Add the shrimp and cook for a minute, or just until the side that is down starts to turn pink. Then flip them over.
Immediately add the wine and stir. Then add the vegetable broth/tomato paste liquid and the capers.
Turn the heat up to medium high and bring to a quick boil. Then lower the heat back down to medium.
In the center of the pan, add a teaspoon at a time of the roux paste and whisk it in quickly to dissolve and distribute.
Keep adding the roux paste a teaspoon at a time, whisking and distributing after every addition, until the sauce starts to look creamy but not thick. It needs to move like a liquid and be pourable, but still have a rich, creamy texture. You will likely not need all the roux.
Taste for seasoning and add salt and pepper if desired. The vegetable broth will usually add enough sodium, so adding salt may not be necessary.
Continue to cook the shrimp in sauce for about 3 minutes more, stirring frequently, to allow the flour to cook through.
Then remove from the stove, garnish with some fresh chopped parsley, and serve.