Perfect Pasta Bake

• 1 tbsp extra virgin olive oil
• 1 brown onion, finely chopped
• 2 cloves garlic, optional
• 100 g broccoli, cut into small florets
• 300 g butternut pumpkin, peeled and cubed
• 85 g baby spinach
• 0.5 cup milk
• 150 g ricotta
• 300 g pasta shells