Combine first 3 ingredients in a medium microwave-safe bowl; microwave at HIGH 2 minutes or until boiling. Stir in onion. Let stand at room temperature for 30 minutes.
Preheat grill to medium-high heat.
Combine mayonnaise and next 4 ingredients (through garlic).
Rub steak evenly with oil; sprinkle with salt and pepper. Place steak on grill rack; grill 5 minutes on each side or until desired degree of doneness. Remove from grill; let stand 5 minutes. Cut steak across the grain into thin slices.
Cut baguette in half lengthwise. Hollow out top and bottom halves of bread, leaving a 1/2-inch-thick shell; reserve torn bread for another use. Place bread, cut sides down, on grill rack; grill 1 minute or until toasted.
Drain onion mixture; discard liquid. Arrange steak evenly over bottom half of baguette; top evenly with onion and arugula. Spread mayonnaise mixture over cut side of top baguette half; place on sandwich. Cut into 4 pieces.
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These were even tastier than I had thought they would be! Had to use ciabatta bread (the grocery store here doesn't carry a decent french baguette) but it worked out fine. We will definitely make this again!! Served as a weekend lunch, so didn't do a side but if serving as a dinner would probably want one (just not sure what health side would go well with it).
These were SO GOOD. Every bit is easy to make, just takes a bit of chopping and prep. Our onions ended up soaking for about an hour, so they were quite tasty. We were able to pack all the ingredients for lunch the next day, and the sandwiches were just as good.
This was PHEnomenal. The bread was soft on the inside and crunchy on the outside, the onions were fresh and crisp tasty and the beef was perfect in this dish. I added some smoked paprika to the aioli and it was sooooo tasty! Served it with some fresh sliced pineapple.