Heat the vegetable oil in a saute pan over high heat. Add the shallot, and red bell pepper, and saute for 1 minute; reduce the heat to medium. Stir the mustard into the cream, and pour the cream into the pan. Allow the liquid to reduce slightly and achieve a thick consistency.
Add in the spinach, and toss until the greens are slightly wilted. Add the tarragon, and lemon juice to the spinach, and toss to combine. Adjust the seasonings with salt, to taste, if desired.