Grilled Smoked Salmon Panini with Red Onion Ribbons

These grilled salmon panini sandwiches are incredibly versatile. Serve them as a hearty lunch, light dinner, or as an appetizer. They can be prepared well in advance, making them perfect for a treat.
Whisk together the vinegar, 2 tablespoons of lemon juice, 2 teaspoons of mustard, and garlic in a bowl. Slowly drizzle with 3 tablespoons of olive oil, whisking constantly until the mixture thickens slightly. Add salt and pepper to taste. Add the red onion and stir to coat. Let stand for 30 minutes (you can prepare it in advance and let it cool).Combine cream cheese and the following 4 ingredients. Add 1/4 teaspoon pepper, remaining 1 tablespoon lemon juice, and remaining 1 teaspoon mustard.Slice the ciabatta horizontally into the top and bottom halves. Spread the cream cheese mixture evenly over both pieces of bread. Spread the salmon evenly over the cheese on the bottom half of the bread; press the salmon firmly against the cheese. Top with spinach and pickled red onion. Put the top half of the bread on a sandwich (you can prepare it in advance if it is tightly wrapped and stored in the refrigerator for 4-6 hours).Cut into 4 separate sandwiches. Preheat the panini or grill over medium-high heat. Brush both sides of each sandwich with olive oil. Cook on the grill for about 5 minutes or until crisp. (If you are making grilled sandwiches, place heavy canned goods on a baking tray.) Cut each sandwich in half diagonally.Note: To serve as an appetizer, cut each grilled sandwich into 3 or 4 rectangles or "fingers".