Heat olive oil in large ovenproof skillet over medium heat.
Add onion and cook, stirring occasionally until softened.
Raise heat to medium-high and add spinach, a handful at a time. Sprinkle with salt and pepper.
Stir and cook just until spinach is wilted and liquid is evaporated.
Combine eggs, Parmesan cheese and season with salt and pepper. Beat well and set aside.
Drain and rinse beans and add to spinach along with basil. Turn heat to low.
Pour egg mixture into skillet, tilting skillet to distribute evenly. Cook until eggs are barely set.
Turn broiler to high; put rack about 4 inches from heat.
Transfer skillet to broiler and cook just until the top turns golden brown. Cut frittata into wedges and serve.