Roasted mushroom ragout with cannellini beans and artichoke hearts

INGREDIENTS

• 1/2 teaspoon red pepper flakes
• red wine or sherry vinegar, to taste
• 1 can (14.5 oz) diced tomatoes (fire-roasted work well here)
• 1/2 small yellow onion, finely chopped (yielding about 1/2 cup)
• 1 can (15 oz) artichoke hearts, drained and roughly chopped
• Kosher salt, to taste
• 2 small garlic cloves, minced
• 2 cups (or 1 can (15 oz) drained and rinsed) cooked cannellini beans
• 2 tablespoons olive oil, plus more for finishing the dish
• 12 large button or cremini mushrooms, cleaned, trimmed, and sliced 1/4" thick (yielding 4 to 5 cups)