Slow Cooker Beef Stew

2 1/2 pounds beef chuck roast cut into 1-inch cubes
1/2 teaspoon black pepper
1/2 teaspoon garlic salt
1/2 teaspoon celery salt
1/4 cup all-purpose flour
3-6 tablespoons olive oil
3 tablespoons cold butter divided
2 cups diced yellow onion
4 cloves garlic minced
1 cup dry red wine like Cabernet or Merlot
4 cups beef broth
2 teaspoons better than bouillon or 2 beef bouillon cubes
2 tablespoons Worcestershire sauce
3 tablespoons tomato paste
5 medium carrots cut into 1-inch chunks
1 pound baby Yukon gold potatoes halved or quartered
2 bay leaves
1 sprig fresh rosemary
1 cup frozen peas
1/4 cup cold water + 3 tablespoons cornstarch optional for thickening
2-3 drops browning sauce like Gravy Master optional

Cut beef into 1-inch cubes, discarding any large fat pieces. Season with pepper, garlic salt and celery salt, then toss with flour to coat.
Heat olive oil in a skillet over medium-high heat. Working in batches, sear beef cubes for 45 seconds per side until browned. Transfer to slow cooker.
Reduce heat to medium and melt 1 tablespoon butter. Cook onions for 5 minutes, then add garlic for 1 minute until fragrant. Deglaze pan with a splash of wine, scraping up any browned bits. Transfer to slow cooker.
Add remaining ingredients except peas, cornstarch slurry, and 2 tablespoons butter to slow cooker. Cook on low for 7 1/2-8 hours or high for 3 1/2-4 hours.
In the last 15 minutes, add peas. Remove bay leaves and rosemary stem.
For a thicker gravy, stir together the cold water and cornstarch, then slowly whisk into the stew.
Turn off heat and swirl in remaining 2 tablespoons cold butter until melted. Add browning sauce if desired.
Serve stew warm over mashed potatoes, egg noodles or with crusty bread.
Beef can be added raw to the slow cooker instead of searing if desired.
Substitute beef broth, red grape juice or stout beer for the red wine if needed, adding 1-2 tablespoons vinegar.
Let stew cool completely before storing in the refrigerator for 3-4 days or freezing for 2-3 months.