Prepare grill or broiler. In a medium bowl, whisk the mustard with the salt and balsamic until blended thoroughly. Slowly drizzle the olive oil, whisking quickly and continuously. The marinade should become thick and emulsified. In a container or large bag, place ribeye steaks and thoroughly coat with marinade. Refrigerate and marinate for 4-6 hours, turning the steaks at least once during the process.
Proceed with grilling or broiling the steaks. After cooking to desired doneness, allow the steaks to rest, loosely covered at room temperature for 10 minutes before serving.
Place spinach in serving bowl. In saucepan, put remaining ingredients, minus the EVOO, and bring to medium heat. Gently warm while whisking. Remove from heat before it simmers. Allow to cool for a minute, and whisk in EVOO to emulsify. Adjust seasoning. Gently dress spinach with warm vinaigrette and top with remaining bacon and shallots.
