Preheat oven to 350 degrees F.
Add one tablespoons of olive oil to a large sauté pan over medium high heat and cook onions until soft and opaque, about five minutes.
Add in potatoes, salt, and pepper to taste; cook covered for about 10 minutes until the potatoes are fork tender. Drain oil and set potato mixture aside.
In a bowl, crack the eggs, add in potato mixture, and whisk until well combined. Heat two tablespoons of butter in a non-stick skillet over medium-high heat; when hot add in egg mixture and reduce heat to medium-low. Cook eggs until they are set on the bottom about 7 minutes, then flip eggs over with a spatula. Cook for another five minutes until the other side of the omelette is set.
While the second side of the omelette sets, cut baguette in half, brush each side of the baguette with olive oil, and toast in the oven for 5-7 minutes until bread id golden brown.
Top one half of the baguette with the Spanish omelette and add the top half to baguette. Cut in quarters and enjoy!