Preheat the oven to 350°F.
Place a steamer basket in a large pot and add 1" of water. Steam the spinach for 2 to 3 minutes, until wilted. Place in a strainer and rinse briefly with cold water.
Squeeze out excess liquid and chop medium-fine.
Heat olive oil over medium heat in a large, heavy skillet and add the onion. Cook, stirring often, until it is tender and beginning to color, about 8 minutes.
Add garlic, thyme, 1/4 tsp salt and some fresh ground pepper. Cook another minute.
Add chopped spinach and stir together. Remove from the heat.
Whisk the eggs in a large bowl. Add 1/4 tsp salt and whisk in the milk.
Stir in the onion and spinach mixture.
Add the cheeses and combine.
Scrape into the tart pan. Place in the oven and bake 40 to 45 minutes, until tart is set and beginning to color on the top. Remove from the heat and allow to cool for at least 15 minutes before cutting.
Serve warm or room temperature.