in a large pan, mix mushrooms, the water, juice, garlic, and 1/2 teaspoons salt. Bring to boiling. Reduce heat. Simmer, covered, for 10 minutes. Drain, cool slightly.
Meanwhile, for marinade, in a large bowl, whisk together oil, vinegar, oregano, mustard, and 1/4 teaspoons salt. Add mushrooms, artichoke hearts, olives, and if desired roasted red pepper. Toss gently to coat. Cover and marinate in the refrigerator for 8 to 24 hours, stirring occasionally.
Let stand at room temperature for 30 minutes before serving. Use a wooden picks or skewers to serve. Makes eight servings.