Lay the peeled and sliced cucumber onto a double layer of paper towels. Sprinkle evenly with the 1 teaspoon of kosher salt and let rest for 5 to 10 minutes.

Wrap the cucumber in the paper towels and aggressively squeeze out the excess moisture. (This osmotic extraction is critical to prevent a watery sauce).

Place the squeezed cucumber slices into the bowl of a food processor along with the peeled garlic cloves and fresh dill. Pulse to combine into a fine mince.

Add the Greek yogurt and fresh lemon juice to the processor. Pulse until the sauce is smoothly emulsified. Season with additional salt and the coarse black pepper. Transfer to a serving bowl and refrigerate.

Organize the 1-inch beef cubes, bell pepper pieces, red onion wedges, and portobello mushrooms on a clean prep board.

Thread the skewers tightly but without crushing the ingredients. Start with 2 pieces of bell pepper, followed by a cube of beef, an onion wedge, and a mushroom.

Repeat the sequence, ending with a final chunk of beef to anchor the skewer. (Yields approximately 6 heavily loaded skewers).

Heat a heavy cast-iron grill pan or outdoor grill over maximum heat.

Aggressively season the assembled skewers on all sides with the kosher salt and coarse black pepper, then drizzle evenly with the olive oil.

Lay the skewers onto the screaming-hot grill. Sear undisturbed for exactly 2 minutes to establish a dark, caramelized crust.

Give the skewers a quarter-turn and sear for another 2 minutes. Repeat until all sides are deeply browned, about 6 to 8 minutes total for a flawless medium-rare center.

Remove from the heat and allow the meat to rest on a warm platter for 3 minutes before serving immediately alongside the chilled tzatziki sauce and warm pita bread.