Step 1 Heat oil in a large saucepan over medium-high heat. Cook onion, carrot, celery and capsicum for 5–7 minutes, until they start to soften. Add oregano, garlic and chilli flakes. Cook, stirring, about 1 minute, until fragrant.
Step 2 Add tomato paste, tomatoes and 1/2 cup cold water. Bring to the boil. Reduce heat to medium-low. Simmer, uncovered, for 15 minutes, until vegetables are cooked through and sauce thickens. Add basil, spinach and beans. Cook, stirring, about 1 minute, until heated through.
Step 3 Meanwhile, combine milk and 1 cup cold water in a medium saucepan. Bring to the boil over medium-high heat. Slowly pour in polenta, stirring constantly. Reduce heat to medium-low. Cook, stirring, for 2 minutes, until thickened. Remove from heat and stir in lemon zest. Stand for 2 minutes, until thickened slightly.
Step 4 Preheat grill to high. Spoon bean mixture into 4 x 2 cup-capacity ovenproof dishes (or an 8 cup-capacity ovenproof dish). Top with polenta and sprinkle with cheese. Grill for 5 minutes, until cheese melts and starts to brown. Serve with green salad.