Combine olive oil, 1/4 cup lemon juice and pomegranate molasses in a screw-top jar. Put mushrooms in a large ceramic bowl, pour over three-quarters of the dressing and stir to coat. Cover and refrigerate 1 hour to allow mushrooms to absorb the dressing.
Preheat grill on high heat. Place capsicums skin side up on a tray, grill for 5-8 minutes until skin is charred. Place in a bowl and cover with plastic wrap. Cool 10 minutes, then peel and discard the skin. Slice capsicums into 2.5cm-thick strips and add to the mushrooms.
Toss tomatoes, parsley and watercress through the mushrooms. Arrange on a platter, drizzle with remaining dressing. Season to taste and serve.