Marinate the steak. Stir together the soy sauce, rice vinegar, honey, minced garlic, and black pepper in a shallow nonreactive dish or food-safe bag. Add the steak and turn to coat. Cover or seal and refrigerate for 90 minutes. Keep it cold; discard the used marinade afterward.

Cook the eggs. While the beef marinates, lower the eggs into gently boiling water and cook for approximately 10–12 minutes, until the whites and yolks are firm. Transfer to an ice bath to cool, then peel and cut into rounds.

Mix the spicy mayo. In a small bowl, stir together the prepared mayonnaise, sriracha, and Dijon mustard. Taste and adjust the heat if desired. Refrigerate until assembly.

Prepare the bread and spinach. Cut the baguette into two equal portions, then split each piece lengthwise. Wash and dry the spinach, if needed. Optionally toast the cut sides of the bread briefly in a clean skillet.

Cook the steak. Remove the beef from the marinade and pat dry with paper towels. Heat the oil in a skillet over medium-high heat. Sear the steak, turning as needed; time depends on its thickness. Check the thickest part using a food thermometer and cook the whole-muscle steak to at least 145°F (63°C). Do not rely solely on a fixed number of minutes.

Rest and slice. Transfer the steak to a clean plate or board and rest for about 5 minutes (at least 3 minutes for food safety). Slice thinly across the grain.

Assemble. Spread spicy mayo on the bread, then layer with spinach, hard-boiled egg rounds, and sliced steak. Close each baguette sandwich and serve promptly.