Cut baguette lengthwise and lightly toast.
Heat skillet over medium-high heat. Add olive oil and onions. Cook for 5 minutes, stirring occasionally.
Separate sirloin slices, add to skillet, and cook for 1-2 minutes to heat through.
Spread dip on both halves of baguette. Layer steak and onions on bottom half, top with spinach and top half of baguette. Slice into 3 sandwiches.
Tip: To make your own spread, mix these ingredients with a fork:
1/4 cup 1/3 less fat whipped cream cheese
2 tbsp. minced roasted red peppers
2 tbsp. shredded Asiago or Parmesan
Salt and pepper, to taste