In a medium skillet over medium-high heat, heat oil. Add onion; season with salt and pepper. Cook, stirring occasionally, until browned in spots, about 10 minutes. Stir in rosemary and sage. Cook, stirring, until fragrant, 1 to 2 minutes. Transfer to a large bowl. Let cool 5 minutes.

Into bowl with onion mixture, crumble sausage and add parsley and Worcestershire; season with salt and pepper and stir to combine.

On a lightly floured surface, roll pastry to a 14" x 12" rectangle. Cut in half lengthwise. Spread 1 ½ tsp. mustard along one long side, leaving a 1" border. Form half of sausage mixture into a 14"-long log on top of mustard. Pull long side closest to you over sausage mixture. Brush pastry on other side of sausage with egg wash. Pull just enough of brushed pastry over to cover remaining sausage by ½". Trim excess and pinch sides together to seal. Arrange sausage roll seam side down on a parchment-lined baking sheet. Repeat with remaining mustard, sausage mixture, and pastry.

Brush ends of rolls with egg wash, then join and pinch ends together to form a circle. Refrigerate until pastry is cool to the touch, about 25 minutes.

Place a rack in center of oven; preheat to 400°. Using a sharp knife, make 1"-long cuts in roll every 1 ½", leaving the inner circle of pastry connected. Twist each cut section sausage side up. Brush pastry with remaining egg wash. Sprinkle with sesame and poppy seeds.

Bake wreath, rotating sheet halfway through, until pastry is puffed and golden brown and sausage is cooked through, 40 to 45 minutes. Let cool 15 minutes.

Transfer wreath to a platter. Serve with chutney alongside.