NOTE: DO NOT ATTEMPT TO GRILL PATTIES ON A BARBECUE RACK OR GAS GRILL. USE AN INDOOR RIDGED FLAT GRILL OR PRESS "GEORGE FOREMAN" CLOSABLE GRILL. I RECOMMEND DOUBLING THE GRAVY RECIPE IF PLANNING TO SERVE GRAVY OVER MASHED POTATOES. THE DIRECTIONS HAVE BEEN CONDENSED SINCE FIRST REVIEW.
MIX the GRILLED SALISBURY STEAKS ingredients in a medium to large mixing bowl (mixture will be soft; patties will shrink considerably while cooking); KNEAD well using hands or mix using a large fork; DIVIDE ground beef mixture to form 4-5 oval-shaped steak patties, about 3/4" thick; PLACE patties onto a platter.
PREHEAT indoor grill to medium-high heat; OIL or spray grill surfaces completely with non-stick cooking spray; GRILL beef patties until sear-marks are obtained; TURN patties and repeat process (if using a press-type closable grill, sear without turning), searing until center of each beef patty is still a little pink, not quite done.
CUT Spanish onion into 1/4" thick, julienne strips to yield 1/2 cup; THINLY slice 4 ounces fresh mushrooms; PREHEAT oven to 350 F if using the OVEN FINISH METHOD (SEE STEP 9).
HEAT 3 tablespoons unsalted butter a large heavy saucepan over medium heat; ADD the onion, mushrooms and mashed garlic clove; SAUTE until onions are light golden and softened; STIR in 1 1/2 tablespoons all-purpose flour; COOK for 1-2 minutes.
QUICKLY whisk the beef broth into the saucepan roux to deglaze, then add the remaining SAVORY MUSHROOM GRAVY ingredients; BRING mixture to a boil over medium-high heat while whisking constantly; REDUCE heat to medium.
WHISK 1/4 cup cold water (or 2 tablespoons water with 2 tablespoons cognac) with 1 1/2 tablespoons cornstarch for the SLURRY in a small mixing bowl, until smooth and cornstarch is completely dissolved; ADD the slurry gradually to gravy base in saucepan, while whisking constantly, until thoroughly blended; COOK gravy until thickened and bubbly; REDUCE heat to low; ADD grilled beef patties, immersing them in gravy.
STOVETOP FINISH METHOD: Simmer over low heat for 10 minutes. CAREFULLY turn steaks with a spatula. Simmer an additional 10 minutes until steaks are steam-cooked and tender.
OVEN FINISH METHOD: Transfer oven-safe large heavy saucepan to preheated 350 F oven. Bake patties uncovered for 20 minutes, immersed in gravy.
SERVE: Transfer steaks onto serving plates with a spatula. Whisk gravy to refresh consistency. Ladle gravy over steaks. Serve with desired side dishes. Enjoy!