To prepare couscous, heat the olive oil in a large nonstick skillet over medium-high heat. Add shallots and beets; sauté 5 minutes or until shallots are tender and just beginning to brown. Stir in couscous; cook 1 minute, stirring frequently. Add water and salt; cover and simmer 8 minutes or until couscous is tender. Remove from heat; stir in spinach. Toss gently until combined and spinach wilts. Keep warm.
To prepare sauce, combine orange juice and next 6 ingredients (through red pepper) in a small saucepan, stirring well with a whisk; bring to a boil over medium-high heat. Cook for 1 minute.
To prepare fish, brush cut sides of fillets with 1/4 cup sauce; place, skin sides up, on grill rack coated with cooking spray. Grill salmon, skin sides up, 2 minutes. Turn salmon fillets; brush with remaining 1/4 cup sauce. Grill 3 minutes or until fish flakes easily when tested with a fork or desired degree of doneness. Serve with couscous and lime wedges, if desired.
Wild Alaskan salmon is in season this time of year, and you can find it in supermarkets and fish markets across the country. Its rich flavor is worth paying a bit more. The ponzu sauce may be made up to a day ahead and refrigerated. Golden beets add sweetness and beautiful color, but don't stain like red beets. Israeli couscous has lovely pearl-like grains that are much larger than regular couscous. Use regular couscous if you can't find Israeli.