Preheat the oven to 425° and drizzle some olive oil on a large sheet pan. Set it aside.
In a large bowl, whisk together the olive oil, lemon juice, mustard, garlic, oregano, salt and pepper.
Toss the russet potatoes in the marinade and then using tongs, transfer them to the baking sheet. Spread them out so they aren't touching. Transfer to the oven and bake for 15 minutes.
While the potatoes are baking, toss the chicken drumsticks in the remaining marinade. Let them sit in the marinade until the potatoes are done. Remove the potatoes from the oven, and turn them oven. Now add the chicken drumsticks to the sheet pan, and add the quartered lemon on the pan.
Brush the chicken and the potatoes with the remaining marinade. Transfer to the oven and bake for 25 minutes or until the chicken is cooked completely.
Removed from the oven, and serve immediately.
Notes
If you don't have oregano, dried basil works well. Chicken thighs will work well too if you don't have drumsticks.