Preheat your oven to 425°F / 220°C.
On a large baking sheet, combine the sliced Brussels sprouts, leeks, olive oil, herbes de Provence, sea salt, and black pepper.
Toss everything together with your hands until the vegetables are evenly coated.
Spread the vegetables in a single, even layer to ensure proper caramelization.
Roast for 20 minutes, stirring halfway through, until the vegetables are tender and have golden-brown, crispy edges.
Don't overcrowd the pan. Use a large baking sheet or two smaller ones to ensure the vegetables roast rather than steam, which is key for getting those crispy, caramelized edges.
Slice the Brussels sprouts and leeks to a similar thickness to promote even cooking throughout the roasting time.