Peel the potatoes. Bring a large pot of salted water to a boil, add potatoes, and cook for 15 minutes until just tender.
While potatoes cook, rinse the cauliflower and cut into small florets. Bring a separate pot of salted water to a boil, add cauliflower, and cook for 8 minutes. Drain thoroughly.
Cut the sausage into strips. Heat a little oil in a large pan over medium-high heat, add sausage and diced onion, and cook for 5-7 minutes, stirring occasionally, until sausage is cooked through (internal temperature 63 °C / 145 °F).
When potatoes are done, drain and slice them. Place sliced potatoes, cauliflower, peas, and cooked sausage in a large baking dish.
In a saucepan, melt the butter over medium heat. Add flour and cook for 1-2 minutes, stirring constantly, until combined. Gradually whisk in the milk and cream while stirring constantly. Bring to a simmer and cook for 2 minutes, stirring occasionally, until slightly thickened. Season with salt, pepper, and nutmeg to taste.
Preheat oven to 175 °C / 347 °F with conventional heat. Pour the béchamel sauce over the vegetables and sausage in the baking dish. Sprinkle grated Emmentaler cheese evenly on top. Bake for 15-20 minutes until the top is golden and bubbling.