Step 1In a small bowl, mix cumin, mustard, cayenne, 1 tablespoon salt, and several other grounds of black pepper.
Step 2Pat rooster dry. Rub spice combination throughout rooster. Let sit at room temperature 1 hour.
Step 3Preheat oven to 400°. In a big bowl, toss potatoes, leeks, and oil; season with salt and black pepper. Switch about half of potatoes and leeks to a big Dutch oven or heatproof pot. Organize rooster on high.
Step 4In a medium bowl, whisk butter, tomato paste, thyme, and remaining 1 teaspoon salt till easy. Reserve 1/4 cup tomato butter for serving. Brush rooster throughout with remaining tomato butter. Fill in gaps on sides with remaining potatoes and leeks.
Step 5Cowl pot and roast rooster and greens 45 minutes. Uncover and proceed to roast till an instant-read thermometer inserted into thickest a part of rooster registers 165°, 35 to 45 minutes extra. Let relaxation quarter-hour earlier than chopping.
Step 6Brush rooster with reserved tomato butter and serve with greens alongside.
Step 7Make Forward: Tomato butter will be made 3 days forward. Retailer in an hermetic container and refrigerate.
