In a medium saucepan, melt the butter over medium heat.
Add the onions and cook, stirring occasionally, until softened.
Stir in the garlic and cook until just fragrant, 30 seconds to 1 minute.
Stir in the couscous and cook, stirring constantly, until lightly toasted all over.
Pour in the water and season with the 1/2 teaspoon salt.
Bring to a boil, then reduce heat to a simmer and cook, stirring occasionally (especially the sides and bottom to prevent sticking), until the water has been absorbed and the couscous is tender, about 10 minutes.
Remove from heat and set aside for 10 minutes to cool.
Transfer the couscous to the large bowl with the vinaigrette and toss well to coat.
Toss in the cucumber, cherry tomatoes, Feta, and chopped parsley.
Adjust salt and pepper to taste.
Serve immediately or refrigerate for up to a day in advance and serve chilled.