Heat butter and 2 tablespoons oil in a large nonstick skillet over medium-high. Add leeks, season with salt and pepper, and cook, stirring often, until translucent, 3 to 4 minutes. Stir in asparagus and 1/2 cup water. Reduce heat to low, cover, and cook until asparagus is bright green and just tender, 3 to 4 minutes. Uncover, increase heat to high, and cook until most of liquid has evaporated, about 1 minute. Transfer to a bowl.