Generously season lamb cubes with salt and pepper; set aside.
In a large skillet over medium-high heat, add oil and when hot, working in batches, add lamb and sear until brown on all sides, about 2 ½ minutes.
Using a slotted spoon, transfer to a bowl and set aside while working with the remaining meat.
Reduce heat to medium and add butter. When it starts sizzling, add onions, carrots and celery; sauté for 5 minutes or until slightly tender.
Add garlic and sauté quickly for 1 minute.
Return lamb pieces back to skillet including any accumulated juices, stir well and sprinkle on flour. Cook for 2 minutes tossing constantly.
Add broth, Worcestershire sauce, rosemary, thyme, and bay leaves. Stir, increase the heat to medium-high and bring to a light boil.
Reduce to medium-low and simmer gently for 20 minutes making sure all the veggies are submerged.
Preheat oven to 350ºF and grease a 3-quart baking dish; set aside.
Transfer the mixture into the prepared baking dish and top with sliced potatoes in a single layer starting from the outside and working inward.
Brush butter evenly on potatoes and sprinkle on some paprika.
Cover tightly with foil and transfer to the preheated oven; bake for 30 minutes.
Uncover, increase heat to 400ºF and bake for an additional 30 minutes or until potatoes are browned and crispy. Remove from the heat and let it rest 10 to 15 minutes before serving.