Place the thick tomato slices on a paper towel-lined plate, then generously season both sides with kosher salt. Let these sit while preparing the rest of the recipe.
In a pan, melt 1 tbsp butter and add 1 tbsp olive oil over medium heat. Place 2 brioche bread slices onto the butter and oil and cook until one side is golden brown, then flip and repeat. Once fully toasted, let the slices cool standing up (leaning against an object) so prevent the moisture from getting trapped.
Pat the halloumi slices dry. Heat a nonstick pan over medium heat and add a small amount of oil as needed. Place the halloumi slices in the pan and allow to cook until crispy and browned, then flip and repeat until both sides of each slice are browned. Set aside to cool.
In a small bowl, add 3 tbsp kewpie mayonnaise, the juice of 1 lemon, and all of the fresh chopped herbs. Add salt and pepper to taste along with 1/2 tsp garlic powder. Mix to combine and adjust seasoning as needed.
Build the sandwich by placing 1 toasted slice of bread down, then slather with a scoop of the herb mayo. Then, add 1 grilled slice of halloumi, followed by a small scoop of herb mayo. Place 1 butter lettuce leaf next, followed by a small amount of herb mayo (acts as a "glue"). Repeat with as many leaves of lettuce as you desire. Next, layer 1-2 slices of tomato (depending on size), followed by 1 more lettuce leaf, 1 more small scoop of herb mayo, then the 2nd slice of halloumi. Spread any remaining mayo on the second bread slice, then top the sandwich with the bread.
Gently press down and carefully slice in half using a serrated bread knife.