Add oil to a small saucepan & heat over high heat until the oil sizzles when a piece of scallion is added. Add green onion, garlic & sesame seeds to a small bowl.
Carefully pour hot oil over green onion so it sizzles & is fragrant, stirring to combine. Let cool & season with salt. Make the scallion topping up to a week in advance & store, refrigerated, in an airtight container until ready to use.
Fill a large cast iron / sauté pan with enough vegetable oil to reach ½-inch up the pan & heat through. Test a corner of one corn tortillas, it should bubble when added to hot oil. Working 1 at a time, add tortilla to hot oil & fry until crispy on both sides (~1 min per side). Remove to a paper towel-lined plate & repeat with remaining tortillas.
Arrange the thin slices of tuna on a plate. Drizzle with the soy sauce & turn to coat. Set aside.
In a medium mixing bowl add the avocado & lime juice. Mash with a fork until smooth. Season to taste with salt.
Evenly spread the avocado purée over the tostada shells. Top with slices of the marinated tuna, & then a spoonful of the scallion sauce, serrano slices & cilantro. Serve immediately.