Marinating chicken thighs: In a mixing bowl rub thighs with salt, lemon juice, 2 tablespoons olive oil, marjoram and pepper. Add the olive pieces and garlic, mix well. Cover and marinate for 6 hours.
Spinach stuffing: In a large pan melt 1 tablespoon butter over medium heat. Add spinach and 3/4 cup dill, cook for 3 to 5 minutes, or until spinach is wilted. Remove from heat. When cool enough to handle squeeze excess moisture from spinach mixture and toss with feta cheese, salt and pepper. Reserve.
Avgolemono sauce: In a small saucepan saute leeks in 2 tablespoons butter. Add chicken stock and simmer for 30 minutes. In a separate saucepan melt 3 tablespoons of butter. Add flour and stir over medium heat for 2 minutes. Remove from heat and strain stock into the roux all at once, whisking until thickened. Reserve. Stuff and saute chicken thighs: Remove olives from the marinade. Chop into small pieces and mix with spinach mixture. Remove chicken thighs from marinade, discard garlic pieces. Smooth out skin of each thigh and create a pocket. Divide spinach mixture and stuff each thigh between skin and meat. Dust with flour on both sides. Heat two skillets over medium-high heat and melt 1 tablespoon of butter in 2 tablespoons of olive oil in each. When foam subsides, add 3 chicken thighs to each skillet. Saute for 5 minutes per side or until golden brown.
Yogurt garnish: In a bowl mix together yogurt and 1/2 tablespoon chopped dill.
To finish: Beat eggs, 1/4 cup lemon juice and remaining 1/4 cup dill into sauce. Taste for seasoning. Warm gently over low heat; do not boil, it will curdle. Drain chicken thighs on paper towels. Place each on dinner plate and pour warmed sauce around. Top with 1 tablespoon of yogurt garnish and a small sprig of dill.