Preheat your oven to 400°F (200°C). Toss the cubed sweet potatoes with olive oil, salt, and pepper, and spread them on a baking sheet. Roast for 25-30 minutes or until tender and slightly caramelized. I've always been apprehensive about using too much oil, but for this dish, the olive oil not only prevents sticking but also enhances the natural sweetness of the potatoes.

While the sweet potatoes are roasting, heat a tablespoon of olive oil in a skillet over medium heat. Add the chopped onion and cook until translucent, about 5 minutes. Stir in the minced garlic, thyme, and rosemary, and cook for another 2 minutes. It's tempting to rush, but slow-cooking the onions until they're caramelized unlocks a depth of flavor that's truly worth the wait.

Once the sweet potatoes are done, reduce the oven temperature to 350°F (175°C). In a large bowl, mash the sweet potatoes with the onion and herb mixture, whole milk, melted butter, half of the cheddar cheese, smoked paprika, salt, and pepper. After adding the spices, give them a quick toast in the pan. This little step awakens their aromatic oils, making the dish even more fragrant.

Transfer the sweet potato mixture to a greased baking dish. When layering the ingredients, press down gently after each addition. This helps in achieving a uniform texture and even cooking.

In a small bowl, mix together the Panko breadcrumbs, remaining cheddar cheese, grated Parmesan, and a drizzle of melted butter. Sprinkle this topping evenly over the sweet potato mixture. The crunch of the breadcrumbs and the richness of the cheese create a contrast that elevates this casserole from comforting to sublime.

Bake in the preheated oven for 20-25 minutes, or until the topping is golden brown and the casserole is bubbling. Allow the casserole to rest for about 10 minutes before serving. This relaxation step lets the flavors meld together and makes it easier to serve.