Heat 1 Tbsp. of the oil in a skillet over medium heat. Add the leeks and garlic and cook until the leeks soften, about 4 to 5 minutes. Add the lemon zest and juice and tarragon and cook until the lemon juice has reduced by half. Add the cream, bring to a simmer, and then reduce the heat to low.
Heat the remaining oil in another large skillet set over medium-high heat. Coat the trout in the flour, shaking off excess. Cook the trout 3-4 minutes per side, or until cooked through and golden. Season the leeks with salt and pepper. Set the trout on plates, spoon over the leeks, garnish, if desired, with lemon wedges and serve.