Heat a couple of tablespoons EVOO, 2 turns of the pan, over medium-high heat in a large soup pot or dutch oven. Add the butter to melt, then add carrots, celery and onions; season with salt, pepper and bay leaf. Cover pot and cook, stirring occasionally, until the carrots soften, 7 to 8 minutes. Sprinkle the flour on the vegetables and stir for a minute, then whisk in the chicken (or vegetable) stock until thickened. Stir in the mustard and peas. For a thinner soup, add 1 to 1 ½ cups water with the stock.