Preheat broiler before warming olive oil in a large skillet over medium-high heat.
Add onions and garlic, cooking until the onions begin to caramelize.
Beat together the egg and milk in a bowl before stirring in the feta cheese, sun-dried tomatoes, olives, basil, spinach, and salt and pepper. Pour mixture over the onions and garlic, reducing heat to medium.
Keep an eye on the edges of the frittata as it cooks, cooking until it begins to set. Slide a spatula around the edge of the frittata to lift it, allowing the uncooked portion to flow underneath.
When the egg is almost cooked, sprinkle on salt, pepper, and the fresh basil. Place the halved cherry tomatoes on top so that each sliced wedge will get at least one. Add a splash of olive oil on top, if you prefer.
Place the skillet in the oven for one to two minutes, or just enough to brown the top of of the frittata. Cut into wedges and serve warm.