For the feta spread: Combine the feta, olive oil, vinegar, mustard, pepper, salt and shallot in the bowl of a food processor. Process until smooth.
For the sandwich: Preheat a grill pan over medium-high heat. Grill the bread until golden brown on both sides, 2 to 3 minutes per side.
Spread each slice of bread with some of the feta spread. Fan the tomatoes over half of the bread slices and sprinkle with half of the kosher salt. Next, fan the cucumber slices over the tomatoes. Scatter the peppers over the cucumbers; top with arugula, basil and cilantro. Sprinkle with the pepper and the remaining salt. Cover with another slice of bread. Cut each sandwich in half and serve.