Bring a large pot of salted water to a boil. Stir in the spinach and cook until bright green, about 30 seconds. Drain and rinse under cold water; squeeze out the excess moisture, then finely chop.
Melt the butter in a large saucepan over medium heat. Add the onion, garlic and 1/2 teaspoon salt and cook until the onion is soft, about 2 minutes. Add the flour and cook, stirring, until lightly toasted, about 1 minute. Whisk in the milk and cook, whisking constantly, until thickened, about 1 minute. Remove from the heat. Stir in the lemon juice, Worcestershire sauce, parmesan and sour cream.
Return the pot to medium heat. Add the spinach, cheddar and artichokes and stir until the cheese melts and the dip is heated through. Serve warm with tortilla chips, salsa and sour cream.