Potato Leek Soup
1 tbsp. olive oil, plus more for serving
4 leeks (white and light green parts only), sliced into half-moons
2 cloves garlic, finely chopped
1 bulb fennel, cut into 1/4-in. pieces
1 stalk celery, thinly sliced
Kosher salt and pepper
3 c. leftover mashed potatoes
6 c. low-sodium chicken broth
4 sprigs fresh thyme
1 bay leaf (optional)
1 tbsp. lemon juice (optional)
Heat oil in large pot on medium. Add leeks, garlic, fennel, and celery, season with 1/4 teaspoon salt and cook, covered, stirring occasionally, until very tender, 13 to 15 minutes.
Add mashed potatoes, broth, thyme, and bay leaf and simmer, stirring occasionally, until heated through, about 5 minutes.
Remove and discard thyme and bay leaf. Using immersion blender (or standard blender, in batches), puree soup until smooth. Stir in lemon juice and serve with cracked pepper and drizzle of oil if desired.