Place the cream in a small pot and reduce by half by gently simmering 20 minutes.
Heat a medium skillet with olive oil over medium heat, add onions and garlic, season with salt and pepper and saute until translucent, 6 to 7 minutes. Drain the spinach by wringing dry in clean kitchen towel. Separate the spinach with your hands and add to onions and garlic. Season spinach with nutmeg then stir in the reduced cream. Adjust flavor with salt and pepper.
Trim the tops off tomatoes and scoop out the seeds. Stuff tomatoes with creamed spinach and top each with 1/4 cup blue cheese. Broil to brown the cheese. Garnish the tomatoes with chives.