Heat up a wide, heavy bottomed pot on medium-high high. Brown sausage on all sides, reduce heat if necessary.
While sausage is browning, prep the next ingredients while the previous is cooking. Dice the onion, mince garlic, then pit and halve sour cherries if using.
Once the sausage is browned and cooked through, remove to a plate and add onion. Cook onion until softened and beginning to caramelize in places. Add garlic and cook for two minutes more. It may take you the entire cook time of the sausage, onion, and garlic to pit the cherries.
Add thyme, bay leaves, lentils, stock, and two teaspoons of salt. Also add dried cranberries if using (fresh sour cherries should be added after the lentils are cooked-see next step). Bring to a boil over high heat, then reduce to a simmer with a lid on. Check after 12 minutes to see how done the lentils are, I prefer the lentils chewy but cook to the doneness you like. Slice sausage and add back to the pot. If the lentils don't have enough liquid, add additional broth 1/2 cup at a time while cooking.
Once the lentils are cooked to your liking, check salt and add more if needed. Turn off heat and add arugula a few large handfuls at a time, stirring to incorporate and wilt. Don't worry, all of the arugula will fit! Add sour cherries if using and lemon juice, mix, ladle out. Crack on black pepper if desired, top with sliced green onion, and enjoy!