Bring the red wine vinegar, mustard, garlic, and capers together in a mixing bowl and whisk until well combined. Add the onions, olives, and the jalapeños from the tuna jar to the bowl and stir to coat; set aside for about 10 minutes to marinate.
When 10 minutes is up, mix in the basil, 2 TBSP of olive oil, and a pinch of salt and pepper.
Carefully slice the baguette in half lengthwise. With a spoon, transfer and spread the olive and onion mixture over the inside of both halves of the bread.
Layer the bottom half of the baguette with sliced tomatoes, roasted peppers, sliced eggs, tuna, and sliced jalapeños. Drizzle with a bit of olive oil and hit it with some salt and pepper.
Carefully close the sandwich and let it sit for about 10 minutes. This allows the dressing from the olive and onion mixture to seep into the bread a bit.
When you're ready to serve, cut the sandwich in half (see my tips and tricks below) and serve.