Place sun-dried tomatoes in a small saucepan and cover with water. Bring to a boil. Remove from heat and let soak until softened, about 30 minutes. Drain and finely chop; set aside.
Meanwhile, bring broth (or water) to a boil in a medium saucepan. Stir in millet and salt; return to a boil. Reduce heat to low, cover and simmer until the millet is tender and liquid is absorbed, 25 to 30 minutes. Let stand, covered, for 10 minutes. Fluff with a fork; transfer to a plate
to cool to room temperature, about 20 minutes.
While the millet cooks, heat 2 teaspoons oil
in a large nonstick skillet
over medium heat. Add onion and cook, stirring often, until softened and light brown, 5 to 7 minutes. Gradually stir in spinach; cover and cook, stirring, until the spinach is wilted, 30 to 60 seconds. Add garlic and cook, stirring, until fragrant, about 1 minute more. Transfer to a plate; let cool for about 10 minutes.
Prepare Olive Ketchup, if using.
Place the millet in a food processor
and pulse to mix lightly. Add the spinach mixture and pulse until coarsely chopped. Transfer to a large bowl; stir in the feta, basil, breadcrumbs, pepper and the reserved sun-dried tomatoes; mix well.
With dampened hands, form the mixture into seven 1/2-inch-thick patties, using about 1/2 cup
Using 2 teaspoons oil per batch, cook 3 to 4 patties at a time in a large nonstick skillet over medium heat until browned and heated through, about 4 minutes per side. Toast English muffins (or buns). Garnish the burgers with arugula, tomatoes and Olive Ketchup, if desired.
Ingredient Note: Millet makes a good grain base here because it gives these burgers added body with a pleasantly mild flavor. Look for it at natural-foods stores.
MAKE AHEAD TIP: Prepare through step 6. Wrap patties individually and refrigerate for up to 2 days or freeze for up to 3 months. Thaw in the refrigerator before cooking.
Fat, total (g)
sat. fat (g)
Monounsaturated fat (g)
Medium-Fat Meat ()
Percent Daily Values are based on a 2,000 calorie diet