This traditional spread resembles a Greek romesco. It goes well with just about anything and is wonderful on its own as a dipping agent for crudités or potato chips. I like to use it anywhere you would use an aioli.
Brush the onion slices with a little olive oil and season with kosher salt and pepper. On a hot grill pan or in a cast-iron skillet, grill the onion until tender and slightly char-marked. Separate into rings.
In a food processor, combine all of the ingredients with the grilled onions and process until very smooth. Taste for seasoning, but be careful not to add too much salt, as the feta is very salty.