Preheat the oven to 350°F (180°C).
Prepare the vegetables (if using): peel the potatoes and cut into 1-2 inches (2.5-5cm) chunks. Peel carrots and cut into 1/4 inch (6mm) slices. Peel onion and cut into quarters. Cut the celery into a couple large chunks.
Add the potatoes, carrots, onion, celery, garlic cloves (unpeeled), herbs, and water to a very large and deep baking dish.
Prepare the chicken legs: in a small bowl, combine olive oil with the spices.
Pat dry chicken legs with paper towels and remove excess fat. Gently loosen the skin without completely removing it, creating enough space for seasoning under the skin. You can use a tablespoon to help with this. Thoroughly season the legs with salt and pepper, ensuring to distribute the seasoning both under the skin and over the skin. Thoroughly rub the seasoned olive oil all over the chicken, including the exterior and underneath the skin.
Now you can place the chicken legs on a wire rack on top of the vegetables or if you don´t have such a baking dish with wire rack, just put the chicken legs directly on the vegetables.
Place the baking dish one shelf below the center of the oven.
Bake for 1 1/2 hours (No need for checking the chicken with a food thermometer, it will be definitely above the ´safe-to-eat temperature´. It will be "overcooked" (in terms of temperature) but this is what makes the best bone-in chicken legs with meat that falls off the bone - just trust me! Please note this method works only for bone-in and skin-on chicken legs, do not use it for breasts or boneless chicken legs).
Remove from the oven, transfer the chicken legs to a plate. Take out the potatoes and carrots to a plate. Strain the liquid that will gather at the bottom of the baking dish. You can discard the onion, garlic, celery, and herbs.
Make the gravy:
Slightly tip the bowl/pot with the strained drippings and using a tablespoon, try to collect the excess fat that will gather on the surface (it will be more translucent than the drippings), discard it (about 8 tablespoons).
In a medium pot, melt the butter. When hot and bubbling, whisk in the flour and corn starch and cook for a minute, stirring.
Add the reserved liquid and Worcestershire sauce, bring to a boil, then cook for a couple of minutes, whisking from time to time, until the sauce is thickened. Try the sauce to check if it needs more seasoning (it won´t probably need salt but it may need a touch of black pepper).
Serve the chicken legs with the gravy, potatoes, and carrots.
Enjoy!