Lamb and lentil cottage pie

Ingredients

• 1 tablespoon olive oil
• 150 grams button mushrooms, thinly sliced
• 1 onion, finely chopped
• 1 carrot, finely chopped
• 1 zucchini, finely chopped
• 2 garlic cloves, crushed
• 400 grams lamb mince
• 400 grams can diced tomatoes
• 1 cup red lentils, rinsed
• 1 cup beef stock
• 1 teaspoon ground cumin
• 1/2 teaspoon smoked paprika
• Potato topping
• 4 medium potatoes, peeled chopped
• 1/5 cup milk
• 1/3 cup grated tasty cheese
• 30 grams butter
• 1 egg yolk