Pat the thawed tuna dry. Divide 1 tablespoon vegetable oil among the four steaks, rubbing it into both sides. Season both sides of the tuna with salt and pepper.
salt and black pepper, 4 tuna steaks
Set a 9 1/2 inch skillet over medium-high heat (a smaller skillet will help keep the oil from dispersing too much). Add 1 tablespoon oil and allow it to heat through. Place the tuna in the skillet and cook for 1 minute on each side, being very careful when you flip it. (Read here about why it is important to have medium-high heat.) 4 tuna steaks, 2 1/2 tablespoons vegetable oil
Remove the tuna and set aside.
Turn the heat on the skillet down to medium. Add the remaining 1/2 tablespoon of oil to the pan. Add the diced jalapeños and cook, stirring occasionally, until they are soft, about 2 minutes.
1 jalapeño pepper
Add the thawed peaches with their juices. Cook for about 3 minutes, stirring occasionally. We are looking for the peaches to really soften and release some of their natural juices.
2 cups frozen peaches
Whisk in the flour. Then add the chicken stock to the pan, stirring it and allowing the sauce to thicken. Taste the sauce and add salt to taste, somewhere between a dash and 1/8 teaspoon. Remove the pan from the heat.
1 1/2 teaspoons flour, 1/2 cup chicken stock
Add the tuna steaks back to the pan, topping them with the sauce. Serve with pickled red onions and chopped cilantro. pickled red onions, cilantro