Mix water and "beef" soup flavoring in a large pot over high heat. Add carrots, celery, potatoes, onion, peas, and corn. Add summer savory, parsley, and black pepper and mix together well. Taste and adjust seasonings as needed.

Mix cornstarch and water together in a small bowl. Add to the pot and mix to combine. Bring to a boil, about 5 minutes. Reduce to a simmer and cook, covered, at least 1 hour.

Meanwhile, heat olive oil in a skillet over medium-high heat. Add seitan pieces and cook until browned, 4 to 5 minutes per side.