Heat 1 tablespoon of canola oil in a large pan over high heat. Add 1/2 of the mushrooms and shallots in 1 layer to sear or brown them evenly. After a few moments, stir and continue to cook for about 5 minutes. Season with salt, freshly ground pepper and add half of the tarragon and the butter and stir to incorporate. Repeat this process again to keep mushrooms from cooking in their own liquid and getting too wet. Serve warm or at room temperature.