Slice the cauliflower down the center to cut it in half.
Cut two equal steaks (one from each half), about 1/2" thick.
Pat dry the steak with a paper towel.
Heat 2 tablespoons canola oil in a 12" cast iron skillet over medium-high heat until it is about to start smoking.
Add the cauliflower steaks and move them around the pan to coat with oil over medium-high heat.
Cook until until golden, then flip and reduce heat to medium. Continue cooking until the cauliflower is tender.
Season on both sides with kosher salt and freshly ground pepper.
Move cauliflower to warm plates.
To make the sauce; add the pine nuts to the pan and stir to toast (quickly), then remove the pan from the heat.
Whisk in 2 tablespoons butter, 2 tablespoons lemon juice, the kalamata olives and chopped parsley until the butter is melted.
Spoon over the cauliflower steaks.
Garnish with parsley and a lemon slice.