Step 1Combine the mustard, egg yolks, green onions, marjoram and lemon juice in a food processor and process until pale and creamy. With the machine running, add the oils in a thin, steady stream through the feed tube until the sauce is thickened.
Step 2Transfer into a large bowl and fold in the whipped cream and capers. Refrigerate until ready to serve.
Step 1Trim the tenderloins of fat and silver skin. Mix the garlic salt, onion salt and pepper in a small bowl. Rub the tenderloins with olive oil and season evenly with spice mixture.
Step 2Place the tenderloins directly over the coals of a hot barbecue. Cover and cook 8 to 10 minutes; turn, cover and cook until an instant-read thermometer inserted into the center registers at least 140 degrees, 8 to 10 minutes longer. If you like your pork more done, aim for 155 to 160 degrees.
Step 3Let the tenderloins rest for 10 minutes before slicing. Slice thinly and top with mustard-caper sauce down the center; pass the rest of the sauce separately. May be served hot or cold, as an appetizer or an entree.