Make marinade. In a large bowl, add the mustard, Yuzu (or lemon juice), olive oil, maple syrup, salt, and pepper. Whisk until combined.
Add chicken. Add the chicken drumsticks directly into the large bowl. Using your hand, mix well until all the chicken pieces are thoroughly coated. Cover and set aside to marinate, at least 30 minutes but preferably overnight (in the fridge) if you can.
Preheat oven. Preheat the oven to 425F/218C. Use a sheet pan with aluminum foil. Or a 9x13" oven-safe casserole dish also works well. Spray with a little oil and use a silicone brush (or your fingers) to coat the oil all over.
Coat with Panko. In a shallow bowl, add the Panko breadcrumbs. Dip the mustard-covered drumsticks, one at a time, into the breadcrumbs. Do not wipe off any excess mustard mixture! Flip until all sides are coated. Press breadcrumbs into the mustard coating. Do not handle too much and be gentle! Add the breaded chicken, in a single layer, to the oiled sheet pan or casserole dish. Repeat until all the chicken pieces are breaded. Lightly spray with oil.
Bake. Transfer the chicken to the oven. Bake until golden brown and cooked through, about 30-35 minutes. Flip the tray or casserole dish about halfway through the cooking time to ensure the chicken browns evenly.
Serve. Remove from the oven and set aside to rest briefly, about 5 minutes. Transfer to a serving bowl and garnish with finely minced parsley, if you like. Serve with mashed potatoes and a fresh green salad. Enjoy!