In a bowl, whisk olive oil, rosemary, thyme, garlic, red pepper flakes, salt, and pepper. Add steak and coat well. Marinate 30 minutes to 2 hours in fridge.
Heat grill to medium-high. Oil grates lightly.
Remove steak from marinade, pat dry. Grill 6 minutes per side for medium-rare (internal temp 135°F). Rest 10 minutes.
Slice steak thinly against the grain.
Grill bread cut-side down 1-2 minutes until toasted.
Drizzle bread with vinaigrette. Layer arugula, steak slices, tomatoes, onion, and Parmesan on bottom half. Close sandwiches.
Cut in half and serve warm or wrap and chill for later.