Heat the 2 Tablespoons of olive oil in a large saucepan, and sauté the onion, celery, and garlic. Add the white pepper and the mushrooms and sauté until most of the liquid expressed by the mushrooms is gone.

In another saucepan, melt the ¹/₄ cup of margarine and add the flour and salt. Cook this mixture for a minute or two, stirring constantly. In my experience this'll burn easily, so use the lowest heat setting you can and stir constantly. Add stock and whisk constantly until the mixture is thoroughly blended. Cook over very low heat until the mixture has thickened slightly, whisking occasionally - about 3 to 4 minutes.

Add the stock-flour mixture to the sautéed mushroom mixture and heat at a simmer, while adding the remaining ingredients, for about 10 to 15 minutes, or until heated through. Do NOT let the mixture boil. I don't know about cooking with milk, but at boiling temperature the soymilk is liable to separate out into little globs. Adjust seasonings to taste.