Heat a large skillet over medium and add the olive oil.
Add the onion and a pinch of salt and cook for 5 minutes or until the onions start to soften.
Reduce the heat to low and add in the garlic. Cook over low heat, stirring frequently, for 10 minutes or until the onions are caramelized and very soft.
Add the defrosted artichoke hearts to the pan along with the red pepper flakes and another pinch of salt and cook for another 5-7 minutes or until the artichokes are tender.
Place the onion and artichoke mixture into the bowl of a food processor and pulse until mostly broken down (or to your desired texture).
Add the cream cheese, yogurt, and mayo to the processor and pulse a few times to incorporate. Add the cheeses and pulse again.
Taste the mixture and season with salt and pepper, as needed.
Preheat the broiler in your oven.
Spread a dollop of the mixture onto each baguette slice and place on a baking sheet. Broil the crostini for 1-2 minutes or until the topping is melted, gooey, and lightly browned in spots. Watch them carefully because this happens fast!
Serve warm or at room temperature, garnished with chopped parsley.
To serve as a dip:
Place the onion-artichoke mixture in a 8 x 8 inch baking dish. Bake for 20-25 minutes in a 350°F oven or until bubbly and golden brown on top. Serve warm, garnished with chopped parsley, with desired dippers.
The crostini topping can be made completely ahead and stored in the fridge. I recommend assembling and broiling just before you are ready to serve.