In a food processor or a large mixing bowl combine (cooked and drained broccoli, (and cooked) cauliflower.
Pulse until softened and broken down. (or use an immersion blender). Add (washed and properly rinsed) spinach, and sun-dried tomatoes, and pulse a couple of times more until combined.
A few chunks here and it is good, don't over pulse until it turns into a puree. You can also use an immersion blender for this.
Add salt, pepper to taste, eggs, ½ cup gluten-free breadcrumbs, panko, and gluten-free flour. Stir with a spatula until combined.
Shape about 2 tablespoon-sized balls.
Prepare a plate/bowl with ¼ cup breadcrumbs and generously dip each vegetable patty in it.
In a nonstick large pan or a grill pan, over medium-high heat add enough vegetable oil (about 1-1 ½ inches).
And once hot cook each patty until golden brown and crispy on both sides (about 5 minutes per side).
You may need to do this in batches as the patties should be apart from each other to prevent sticking.
Transfer to a large plate lined with a paper kitchen towel to remove the extra moisture and oil. Cover with aluminum foil while you finish cooking the rest of the patties.
You should end up with a nice textured, crispy exterior firm burger. Unless you make them very thin, but in this case, there is a slight risk of them falling apart.
Serve the vegetarian patties warm as a delicious side dish, with your favorite dipping sauce, or as a part of the main meal with a portion of rice, mashed potatoes, and gravy or white sauce.