Spread the garlic butter mixture on the baguette halves. Toast in a 350°F (175°C) oven for about 15 minutes or until golden. Set it aside.
Heat a skillet on high heat and add 1 tbsp of olive oil. Once the pan is smoking, sear the steak—flip every 30 seconds for about 2 minutes and 30 seconds for medium-rare.
Reduce the heat and add butter, garlic cloves, and thyme. Baste the steak in melted butter for 2 minutes.
Remove from heat and let the steak rest for 5–10 minutes. Slice thinly against the grain.
Spread lemon mayo onto toasted garlic bread. Layer with sliced steak, arugula, and tomato. Close, slice, and serve!