Preheat oven to 350F (180C). Steam the fresh spinach until just wilted. Press the water out and set aside.
Heat the oil in a small cast iron skillet over medium heat and cook the onions until soft but not brown. In a large bowl, mix ricotta, eggs, cream, flour, melted butter and dill. Season with salt and pepper. Add the prepared spinach and tomatoes. Mix thoroughly and pour into the skillet over the onions.
Sprinkle Parmesan or Montassio cheese over the top and bake until set, about 45 minutes or until top browns. Let the quiche cool for 5 or 10 minutes before serving.