Saute onion, green bell pepper, celery, garlic, salt, pepper, paprika, thyme, and mustard in butter in large heavy skillet over medium-high heat for 4-5 minutes or until vegetables soften; remove from heat.
Place roast in shallow roasting pan; fat side up.
Make several slits with sharp knife in the top of the roast, cutting downward, but not through to the bottom.
Stuff pockets with the vegetable filling.
Roast in preheated oven for 1 hours or more, or until a meat thermometer reads the temperature you desire (160 degrees-for medium; 170 degrees for well-done).
Remove roast from pan; keep warm; reserve pan drippings.
To prepare gingersnap gravy, pour off pan juices into a 1 cup measure; skim off fat.
Add enough water to make 1 cup and pour back into the pan.
Set pan on top of the stove over medium-high heat, and simmer for 2 minutes, scraping brown bits from the bottom of the pan; remove from heat.
Saute chopped onion, chopped celery, minced garlic in butter in large skillet for 4-6 minutes.