Blanch the spinach in boiling water for 1 minute. Drain and refresh in cold water and blend in a mixer to a smooth purée. Keep aside. Heat the butter in a broad non-stick pan, add the garlic, spring onions whites and greens and sauté on a medium flame for few minutes. Add the red capsicum, mix well and sauté on a medium flame for 1 to 2 minutes. Add the prepared spinach purée, mix well and cook on a medium flame for 1 to 2 minutes, while stirring occasionally. Add the cheese, milk and salt, mix well and cook on a medium flame for 1 minute, while stirring occasionally. Add the spaghetti, oregano and chilli flakes, toss well and cook on a medium flame for 2 to 3 minutes. Serve immediately.