Step 1Spray a large saucepan with oil and place over a medium heat. Cook onion, carrot and celery, stirring occasionally, for 6-8 minutes or until vegetables are softened. Add garlic and cook, stirring, for 1 more minute. Add mince and cook, stirring, for 5 minutes or until browned.
Step 2Add tomato paste and cook, stirring, for 2 minutes. Add tomatoes and ¼ cup water. Cook, stirring occasionally, for 10 minutes or until sauce thickens. Add spinach and stir through. Remove from heat and set aside.
Step 3Preheat oven to 180°C. Lightly spray an 8-cup-capacity (25cm x 18cm x 6cm) ovenproof dish with oil. Place kumara in a large microwave-proof dish with 2 tablespoons water. Cover and microwave for 4-5 minutes on high or until just tender. Drain.
Step 4Whisk ricotta cheese, egg and milk in a large bowl until smooth.
Step 5To assemble lasagne, spread one-third of the kumara to cover base of prepared dish. Top with half the mince mixture. Repeat with another kumara layer and remaining mince. Finish with a kumara layer then spread with ricotta mixture. Bake for 40-45 minutes or until top is golden and bubbling. Sit lasagne for 5 minutes before serving.