Beef and Veggie Smashed Potato

Ingredients

• • 2 russet potatoes, scrubbed clean
  • 4 oz (115 g) white mushrooms, sliced
  • 1 small onion, cut into thin wedges
  • 1/2 green bell pepper, seeded and cut into strips
  • 1/4 cup (55 g) butter
  • 8 oz (225 g) thinly sliced fondue beef
  • 1/4 cup (60 ml) beef or chicken broth
  • 1 tsp (5 ml) Dijon mustard
  • 1/4 cup (25 g) mozzarella cheese, grated (or more to taste)
  • Sour cream, to taste
  • Knob of butter (optional)