In a large bowl, whisk the eggs until well beaten. Add the quinoa, milk and a generous amount of seasoning. Add in half of the cheese, a few sprigs of chopped basil and the spinach. Mix well to combine then pour in the slow cooker. Arrange the tomato slices evenly over the surface then top with the remaining cheese. Cook on high for 2 hours. Scatter over some more basil leaves then serve.